Enhance support for @DataSchema in gradle projects

[koperagen]

Currently, a KSP preprocessor generates DataSchema-boilerplate. We want to address the following:

1. You need to build a project to generate boilerplate. We want the same experience as kotlinx.serialization (generate as you type, rename / move refactorings)
2. Better error reporting. Report warning for local classes annotated with @DataSchema
3. Add companion object to a data schema interface automatically https://github.com/Kotlin/dataframe/issues/113
4. Resolution of the generated code without extra gradle configurations on the user side.

If 1 and 4 are solved on the KSP side with the release of K2, then we only need a small compiler plugin for 2 and 3. Otherwise, codegen should be moved to a compiler plugin as well.