Re-writing accessors transformer using the new AGP API

[nhachicha]

• [x] Migrating the Realm accessor Transformer to use the new AHP transformer API
• [x] Migrated Gradle plugin from Groovy to Kotlin.
• [ ] Re-writing the internal object server transformer.

[nhachicha]

@rorbech I updated the current WIP

• Currently, the JNI generated header files (.h) are not created if we invoke assemble, this might be caused by the various bumps to Gradle and AGP ... if you run compileBaseReleaseJavaWithJavac for instance then it will compile and generate the headers.
• The internal transformer (the one that strips the ObjectServer symbols) needs to be re-written similarly to the accessor one.
• I updated some tests projects to run the new transformer against them
• We need to create a feature branch and release from it (probably using this template https://github.com/realm/realm-java/pull/7704

For reference:

• https://issuetracker.google.com/issues/237190740#comment4

[rorbech]

I manually verified that all our examples are running with the current state. Also verified that updating model and other sources is properly rebuilt in incremental builds for a couple of the examples.