Document the things needed to add an extension with type mappings

Based on the excellent work of Shay Rojanski and Steve Rasmusmen to integrate NodaTime, I tried to do the same with Ulid.

The wip repo is here https://github.com/linoafr/UlidIntegration with unit tests if this can help.

Ulid api works almost like Guid api

```csharp
Ulid.NewUlid() // Produce 01FJ407WVXP0N8JNEM763SW753
Ulid.Empty // Produce 00000000000000000000000000
Ulid.MinValue // Produce 00000000000000000000000000
Ulid.MaxValue // Produce 76EZ91ZPZZZZZZZZZZZZZZZZZZ
```

Ulid can be stored in a database as char(26) or binary(16).

I think the relevant code is here for char(26).

```csharp
internal class UlidToStringValueConverter : ValueConverter
{
private static readonly ConverterMappingHints DefaultHints = new(size: 26);

public UlidToStringValueConverter() : this(null)
{
}

internal UlidToStringValueConverter(ConverterMappingHints? mappingHints = null)
: base(
convertToProviderExpression: x => x.ToString(),
convertFromProviderExpression: x => Ulid.Parse(x),
mappingHints: DefaultHints.With(mappingHints))
{
}
}

internal class UlidToStringTypeMapping : RelationalTypeMapping
{
private static readonly UlidToStringValueConverter ValueConverter = new();

public UlidToStringTypeMapping()
: base(new RelationalTypeMappingParameters(new CoreTypeMappingParameters(
typeof(Ulid),
ValueConverter),
"char",
StoreTypePostfix.Size,
System.Data.DbType.AnsiStringFixedLength,
unicode: false,
size: 26,
fixedLength: true))
{
}

protected override RelationalTypeMapping Clone(RelationalTypeMappingParameters parameters)
=> new UlidToStringTypeMapping();
}

internal class UlidToStringTypeMappingSourcePlugin : IRelationalTypeMappingSourcePlugin
{
public RelationalTypeMapping? FindMapping(in RelationalTypeMappingInfo mappingInfo)
{
if (mappingInfo.ClrType == typeof(Ulid))
{
return new UlidToStringTypeMapping();
}

return null;
}
}

```

## This query is working (SqlServer and Npgsql)

```csharp
var myId = Ulid.NewUlid();
var e = db.MyEntity.SingleOrDefaultAsync(p => p.Id == myId);
```

## This queries are not working (SqlServer and Npgsql)

### Ulid.NewUlid()

```csharp
var e = db.MyEntity.SingleOrDefaultAsync(p => p.Id == Ulid.NewUlid());
```

```
Message: 
Npgsql.PostgresException : 42601: syntax error at or near "FJ2B2J3BMN1AVKT6SBA15D8R"

POSITION: 64

Stack Trace: 
NpgsqlConnector.g__ReadMessageLong|215_0(NpgsqlConnector connector, Boolean async, DataRowLoadingMode dataRowLoadingMode, Boolean readingNotifications, Boolean isReadingPrependedMessage)
NpgsqlDataReader.NextResult(Boolean async, Boolean isConsuming, CancellationToken cancellationToken)
NpgsqlCommand.ExecuteReader(CommandBehavior behavior, Boolean async, CancellationToken cancellationToken)
NpgsqlCommand.ExecuteReader(CommandBehavior behavior, Boolean async, CancellationToken cancellationToken)
NpgsqlCommand.ExecuteDbDataReaderAsync(CommandBehavior behavior, CancellationToken cancellationToken)
RelationalCommand.ExecuteReaderAsync(RelationalCommandParameterObject parameterObject, CancellationToken cancellationToken)
RelationalCommand.ExecuteReaderAsync(RelationalCommandParameterObject parameterObject, CancellationToken cancellationToken)
AsyncEnumerator.InitializeReaderAsync(AsyncEnumerator enumerator, CancellationToken cancellationToken)
NpgsqlExecutionStrategy.ExecuteAsync[TState,TResult](TState state, Func`4 operation, Func`4 verifySucceeded, CancellationToken cancellationToken)
AsyncEnumerator.MoveNextAsync()
ShapedQueryCompilingExpressionVisitor.SingleOrDefaultAsync[TSource](IAsyncEnumerable`1 asyncEnumerable, CancellationToken cancellationToken)
ShapedQueryCompilingExpressionVisitor.SingleOrDefaultAsync[TSource](IAsyncEnumerable`1 asyncEnumerable, CancellationToken cancellationToken)
UlidTests.Query_with_ulid_new_method() line 45
--- End of stack trace from previous location ---
```

### Ulid.MinValue (Unit test is green for SqlServer but I have a project with this error message "Error converting data type varchar to numeric")

```csharp
var e = db.MyEntity.SingleOrDefaultAsync(p => p.Id == Ulid.MinValue);
```

```
Message: 
Npgsql.PostgresException : 42883: operator does not exist: character = integer

POSITION: 60

Stack Trace: 
NpgsqlConnector.g__ReadMessageLong|215_0(NpgsqlConnector connector, Boolean async, DataRowLoadingMode dataRowLoadingMode, Boolean readingNotifications, Boolean isReadingPrependedMessage)
NpgsqlDataReader.NextResult(Boolean async, Boolean isConsuming, CancellationToken cancellationToken)
NpgsqlCommand.ExecuteReader(CommandBehavior behavior, Boolean async, CancellationToken cancellationToken)
NpgsqlCommand.ExecuteReader(CommandBehavior behavior, Boolean async, CancellationToken cancellationToken)
NpgsqlCommand.ExecuteDbDataReaderAsync(CommandBehavior behavior, CancellationToken cancellationToken)
RelationalCommand.ExecuteReaderAsync(RelationalCommandParameterObject parameterObject, CancellationToken cancellationToken)
RelationalCommand.ExecuteReaderAsync(RelationalCommandParameterObject parameterObject, CancellationToken cancellationToken)
AsyncEnumerator.InitializeReaderAsync(AsyncEnumerator enumerator, CancellationToken cancellationToken)
NpgsqlExecutionStrategy.ExecuteAsync[TState,TResult](TState state, Func`4 operation, Func`4 verifySucceeded, CancellationToken cancellationToken)
AsyncEnumerator.MoveNextAsync()
ShapedQueryCompilingExpressionVisitor.SingleOrDefaultAsync[TSource](IAsyncEnumerable`1 asyncEnumerable, CancellationToken cancellationToken)
ShapedQueryCompilingExpressionVisitor.SingleOrDefaultAsync[TSource](IAsyncEnumerable`1 asyncEnumerable, CancellationToken cancellationToken)
UlidTests.Query_with_ulid_minvalue() line 52
--- End of stack trace from previous location ---
```

### Ulid.MaxValue

```csharp
var e = db.MyEntity.SingleOrDefaultAsync(p => p.Id == Ulid.MaxValue);
```

```
Message: 
Npgsql.PostgresException : 42601: syntax error at or near "EZ91ZPZZZZZZZZZZZZZZZZZZ"

POSITION: 64

Stack Trace: 
NpgsqlConnector.g__ReadMessageLong|215_0(NpgsqlConnector connector, Boolean async, DataRowLoadingMode dataRowLoadingMode, Boolean readingNotifications, Boolean isReadingPrependedMessage)
NpgsqlDataReader.NextResult(Boolean async, Boolean isConsuming, CancellationToken cancellationToken)
NpgsqlCommand.ExecuteReader(CommandBehavior behavior, Boolean async, CancellationToken cancellationToken)
NpgsqlCommand.ExecuteReader(CommandBehavior behavior, Boolean async, CancellationToken cancellationToken)
NpgsqlCommand.ExecuteDbDataReaderAsync(CommandBehavior behavior, CancellationToken cancellationToken)
RelationalCommand.ExecuteReaderAsync(RelationalCommandParameterObject parameterObject, CancellationToken cancellationToken)
RelationalCommand.ExecuteReaderAsync(RelationalCommandParameterObject parameterObject, CancellationToken cancellationToken)
AsyncEnumerator.InitializeReaderAsync(AsyncEnumerator enumerator, CancellationToken cancellationToken)
NpgsqlExecutionStrategy.ExecuteAsync[TState,TResult](TState state, Func`4 operation, Func`4 verifySucceeded, CancellationToken cancellationToken)
AsyncEnumerator.MoveNextAsync()
ShapedQueryCompilingExpressionVisitor.SingleOrDefaultAsync[TSource](IAsyncEnumerable`1 asyncEnumerable, CancellationToken cancellationToken)
ShapedQueryCompilingExpressionVisitor.SingleOrDefaultAsync[TSource](IAsyncEnumerable`1 asyncEnumerable, CancellationToken cancellationToken)
UlidTests.Query_with_ulid_maxvalue() line 59
--- End of stack trace from previous location ---
```

### Include provider and version information

EF Core version: 6.0.0-rc.2.21480.5
Database provider: SqlServer & Npgsql
Target framework: 6.0.0-rc.2.21480.5
Operating system: Windows 10
IDE: Visual Studio 2022 17.0.0 Preview 5.0

Thanks in advance for your help