Cherry-pick #66 to 7.x: Add cherrypick_pr dev-tools helper.

[blakerouse]

Cherry-pick of PR #66 to 7.x branch. Original message:

What does this PR do?

Adds the dev-tools/cherrypick_pr helper script to help cherrypick changes from master to 7.x branches.

Why is it important?

7.x branch is now created for release manager building, so we need to start backporting any changes to that branch.

Checklist

• My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

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Build stats

• Build Cause: Pull request #67 opened

• • Start Time: 2021-01-20T15:01:09.668+0000

• Duration: 4 min 46 sec

• Commit: 397ce3b

Test stats 🧪

Test	Results
Failed	0
Passed	48
Skipped	0
Total	48