Upgrade source generators to the more efficient incremental generator style

[TheCakeMonster]

The SerializationPartialsGenerator type was created to automatically implement the IMobileObject interface on POCO types and does so by implementing the ISourceGenerator interface. In .NET 6, Microsoft added a newer generator API that offers better performance on larger code bases - incremental source generators. Incremental source generators can reduce load on the build infrastructure during build and development, reducing the risk that the generator will slow developer activities.

Upgrading from implementing ISourceGenerator to instead implement IIncrementalGenerator could be beneficial for users of CSLA who develop extremely large codebases.

This would affect the SerializationPartialsGenerator class and associated types.

The difference for a single generator is probably quite small, so this is a low priority task. This task would become more relevant if we added more source generators.