[Transformer] Add support for incremental builds

[clementetb]

Add support for incremental builds to the new transformer API with Gradle incremental tasks.

The logic is similar to the previous incremental builds: we process and update only the modified files, but now the source of changes is the gradle task instead of the Gradle plugin.

One particularity of the new transformer API is that before the classes were outputted in a plain directory, whereas now they are in a single Jar file.

Incremental builds can be disabled by setting the gradle property io.realm.disableIncrementalBuilds to true.

Tested manually with a sample app with:

• A library module.
• Resources.
• Enabling/Disabling incremental builds with the gradle setting.
• Validating that deleted classes are removed from the output Jar.

TODO

• [x] Changelog
• [x] Review documentation
• [x] Add build flag to disable incremental builds

[clementetb]

@rorbech validated that configuration-cache works normally.