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
tree: 9ddb704418bbbfc3517167d29b13592a13012483
  1. .asf.yaml
  2. .gitignore
  3. README.md
  4. cherry-pick/
  5. diff-only/
  6. pulsarbot/
  7. setup-maven/
README.md

Apache Pulsar Test Infrastructure

This repo is used for keeping all the scripts and github actions for running Pulsar test infrastructure.

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