ICU-21755 commit checker: add support for COMMIT_METADATA.md file

ICU-21755 commit checker: add support for COMMIT_METADATA.md file

- new option, --commit-metadata= with path to metadata file
- new option, --fix-version=41 (used for SKIP sections)
- scaffolding for 'bad commits' list
- new module CommitMetadata with unit tests
- sample file format TEST_COMMIT_METADATA.md
- such commits are skipped
- refactored the commit skipping part (formerly used for cherry pick skips)
- add a report section for skipped commits
- add a cache for JIRA queries (for dev use): --cache-for-dev "/tmp/cldr-commit-cache" - optional
- add an 'excluded commits' section at the bottom
- make sure commit metadata is used to update ticket IDs and messages.