build: update monorepo-migration scripts

[chingor13]

Rebase of #11977

• split integration tests from the bulk integration tests that only run when that library is modified
• improve logic for managing intra-repository versions (remove dependencies on gapic-only boms)
• rename customNative and native-tests profiles for customizing graalvm tests (you can override settings in the native maven profile)
• update .repo-metadata repo and repo_short entries
• adds TEST_WITH_EXISTING mode for copying new integration test config to existing job for testing (less repository disruption)
• preserves custom kokoro scripts
• adds SKIP_CI_WORKFLOW option for skipping copy of ci.yaml for split repo
• adds ability to re-apply custom changes from ~/<repo>.diff