Implement support for the new SQL Server native JSON type

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SQL Azure now has the new JSON type as a public preview (docs); the type will also appear in the next on-premise SQL Server version. We should ensure support for this in EF. We likely want to wait until some sort of on-premise SQL Server supports this.

External things we depend on

• [ ] We're blocked until we're provided a SqlClient version that supports the new JSON type (tracking issue).
  • We can release without a dependency on the new SqlClient. We just need the SqlDbType value from the BCL.
• SqlDbType.Json is being exposed in the runtime for .NET 9.0 (issue). EF will presumably need to use that to specify to SqlClient to send the new JSON type.

TODO

• Implement type mapping support for mapping .NET string to the new json type.
• Allow users to map primitive collections to the new JSON type (by explicitly configuring the column type - we'll consider switching the default later).
• Json: add ability to specify type of the json column #28452 (to allow users to map owned entity types to the new JSON type).
• Test-wise, we'd ideally duplicate the PrimitiveCollections and other JSON-related tests to also execute when configured with json. Though note that the JSON type is currently only available in SQL Azure, and we don't currently test against it, as there's no local emulator version.

/cc @saurabh500

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The JSON feature seems to be out (as preview) on Azure SQL, see docs.

Some things to think about:

• Should/can we switch to mapping to the new type by default (as opposed to nvarchar(max))?
  • The type is currently only available on Azure SQL, and not on any on-prem SQL Server version. If we decide to change the default, we can do that only when the user uses the new UseAzureSql() instead of UseSqlServer() (not yet implemented, see Introduce UseAzureSql/UseAzureSynapse as alternatives to UseSqlServer #33816).
  • We'd need to carefully think of the migration path for users with existing databases, e.g. is it possibly to automatically migrate existing nvarchar(max) columns to the new json type (and is this something we actually want).
  • This would apply to both owned entity types and primitive collections (which are mapped to JSON columns).
  • Specifically for complex types, we don't currently support mapping these to JSON so it may be easier to map to the new JSON type (no breaking change).