Added the cherry-pick automation (#1728)
The process of the cherry-pick automation is as follows:
1) index.py is triggered with the inputs by action.yml
2) Check if the PR/issue is closed
3) Check if the commit id exists and if it was done by
"copybara-service[bot]". If yes, then retrieve the commit id
4) Check if approved by reviewers and get the GitHub ID's of the
reviewers
5) Get labels
6) Perform cherry-pick
7) Create a PR
8) Issue a comment in the milestoned issue whether or not the
cherry-pick was performed
4 files changed
