Add a github action for automatically cherry-pick PRs (#13)
**Motivation**
We want to make the pulsar can cherry-pick automatically. This action will make a closed PR can cherry-pick by the command `/pulsarbot cherry-pick to branch-X.Y`.
When a PR is merged, the bot will say,
```
Hey. If you want to cherry-pick this pr to a target branch, please comments '/pulsarbot, /pulsarbot cherry-pick to branch-X.Y'.
```
Then you can comment /pulsarbot cherry-pick to branch-X.Y, the bot will open a new PR for cherry-picking the closed PR.
If we open a new PR to do the cherry-pick, we can also use the CI to check the branch.
5 files changed
