Support OpenApi Schema Service in CommunityToolkit.Datasync.Server.OpenApi

[adrianhall]

In .NET 9, we can't inject the entity schema into the OpenApi definition when using the new .NET9 AddOpenApi / MapOpenApi.

This is because:

• OpenApiSchemaService is internal sealed and in a different assmebly (preventing reflection)
• We cannot add a [Consumes<T>] or [ProducesResponseType<T>] to the controller method (since TableController is generic)

In Swashbuckle and NSwag, we generate the schema for the entity type, but this is not possible in .NET 9 OpenApi.

Once Microsoft.AspNetCore.OpenApi allows us to inject schema (or C# allows us to add attributes based on a generic type), we can add such functionality to the OpenApi transformers.