[Enhancement]: Support XML data type for MSSQL

# XML data type

In SQL Server and Azure SQL Database, the `XML` data type stores structured XML content. Unlike the `JSON` data type, `XML` has been supported since SQL Server 2005 and includes native indexing, query, and validation capabilities.

```sql
CREATE TABLE documents (
id INT PRIMARY KEY,
metadata XML -- a native XML document
)
```

## FOR JSON support

When used with `FOR JSON`, SQL Server will **emit the XML content as a string**, not a parsed structure. This is because XML is not directly translatable to JSON without transformation.

```sql
SELECT id, metadata FROM documents FOR JSON AUTO;
```

Returns:

```json
[
{
"id": 1,
"metadata": "reportadmin"
}
]
```

## Inserting XML

```sql
INSERT INTO documents (id, metadata)
VALUES (
1,
'reportadmin'
);
```

* The XML must be passed as a valid string literal.
* SQL Server will parse and validate the XML at runtime.
* You can define an XML schema collection for strict validation if needed.

## Data API builder behavior

Data API builder (DAB) should treat the `XML` column as a **string** in both directions.

### Query operations

DAB will expose XML values as string content in REST responses:

```json
{
"value": [
{
"id": 1,
"metadata": "reportadmin"
}
]
}
```

### Mutation operations

Whether creating or updating, input must be a string containing valid XML:

```http
POST /documents
Content-Type: application/json

{
"id": 2,
"metadata": "memoguest"
}
```

### XML Considerations

1. SQL Server will validate the string as well-formed XML before storing.
2. DAB does not parse or format XML—it treats the field as a text payload.
3. XML schema collections can be applied to restrict content if required.