Update controller source generator to incremental

[bkoelman]

WebAPI controller classes in JsonApiDotNetCore are generated at compile-time using our source generator. We can potentially improve the IDE responsiveness by switching to the newer incremental roslyn API. Note this does not affect build time.

There's a great series with all the gotchas at https://andrewlock.net/creating-a-source-generator-part-9-avoiding-performance-pitfalls-in-incremental-generators. Requires. NET 7 or higher.