Add magic phrase support to rerun tests

[SinaChavoshi]

Currently to re run tests need to submit an empty commit

git commit -m "retest" --allow-empty and then 
git push <branch-name>

This is a request for a nice to have feature to enable rerun of tests with a magic phrase such as "/retest" or "/rerun tests".

[pavithrasv]

Adding this info here for reference: We can rerun github actions on the github check tab of a PR. Note from @ucdmkt : for Kokoro test we can add kokoro:run label.