feat(multi-module): Introduce SentryRootPlugin with App and Lib subplugins

[romtsn]

#skip-changelog

:scroll: Description

• Introduce a new io.sentry.gradle plugin that has to be applied on the root project level. The plugin then:
  • Goes over subprojects and applies the respective subplugins depending on what plugins are currently applied to the gradle project
  • AppSubplugin implementation remained untouched and just works, however I will refactor and extract some things later on
  • LibSubplugin implementation to follow
  • Checks if the io.sentry.android.gradle plugin is in place and throws as they are incompatible and can't be used together
• Deprecate io.sentry.android.gradle and print a message to switch over to the new root plugin
  • It also delegates to the same AppSubplugin that is used by the new root plugin
• Factor out common things into a separate file that we share among the plugins

:bulb: Motivation and Context

Closes #800

:green_heart: How did you test it?

So far just manually, will add/convert existing tests later

:pencil: Checklist

• [x] I reviewed the submitted code
• [] I added tests to verify the changes
• [ ] I updated the docs if needed
• [ ] No breaking changes

:crystal_ball: Next steps

• Make LibSubplugin do the actual work
  • also investigate if we can reuse the same one for all kinds of libs, not only android
• Make RootPlugin collect sources from all modules into a single bundle and upload it
• Introduce new SentryRootExtension and SentryProjectExtension to allow configuration per-project or globally on a root project level
• Convert tests to use the root plugin and add a single test that tests the deprecated one

[github-actions[bot]]

	Messages
:book:	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

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