Support merging existing resources

[ZacSweers]

Requires google/ksp#431

[bubenheimer]

I think the auto-service-ksp processor code currently does not even merge its own resources, i.e. multiple rounds would overwrite each other or it would throw. It looks like there is an intention to gather everything for a later write, like SymbolProcessor.finish(), but then, actually, writes are not being deferred. Or does CodeGenerator.createNewFile() actually append? Maybe I missed something or it's left incomplete because of this issue. Just thought I'd share my findings from studying the code.

https://github.com/ZacSweers/auto-service-ksp/blob/main/processor/src/main/kotlin/dev/zacsweers/autoservice/ksp/AutoServiceSymbolProcessor.kt