Json Serialiser doesn't handle structs well and ignores type converters

Hello,

I have some wrapper types that I want to use with GraphQL, which look like this:

```csharp
public struct ProductId(Guid value) {
public Guid Value => value;

public override string ToString() => value.ToString();
public override bool Equals(object? obj) => obj is ProductId other && other.Value == value;
public override int GetHashCode() => value.GetHashCode();
public static bool operator ==(ProductId left, ProductId right) => left.Value == right.Value;
public static bool operator !=(ProductId left, ProductId right) => !(left == right);
}
```

I also provide `JsonConverters` for these types, which look similar to:

```csharp
class ProductIdSystemTextJsonConverter : JsonConverter
{
public override ProductId Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) =>
new Foo(Guid.Parse(reader.GetString()!));

public override void Write(Utf8JsonWriter writer, ProductId value, JsonSerializerOptions options) =>
writer.WriteStringValue(value.Value);
}
```

I register the converters in the client:

```csharp
var serialiser = new SystemTextJsonSerializer
{
Options =
{
PropertyNameCaseInsensitive = true
}
};
serialiser.Options.Converter.Add(new ProductIdTypeConverter());
```

But when deserialising the GraphQL response I get this error:

```
System.InvalidOperationException
Sequence contains no matching element
at System.Linq.ThrowHelper.ThrowNoMatchException()
at System.Linq.Enumerable.First[TSource](IEnumerable`1 source, Func`2 predicate)
at GraphQL.Client.Serializer.SystemTextJson.ImmutableConverter.Read(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options) in /_/src/GraphQL.Client.Serializer.SystemTextJson/ImmutableConverter.cs:line 80
```

I do not quite understand why `ImmutableConverter` was chosen and not the specific one that I registered...

I validated the approach, and it works if I create my wrapper types as `record`s somehow:

```csharp
public readonly record struct ProductId(Guid Value)
{
public override string ToString() => Value.ToString();
}
```

But the goal is to make it work with plain structs and have libraries like https://github.com/lucasteles/Strongly to generate wrapper types, Json converters, and all the required boilerplate around them.

I believe that fixing the "plain structs" misbehavior would allow it to work rather smoothly.