[FEAT] Thank you note on PR getting merged

[Varun-Kolanu]

Is your feature request related to a problem? Please describe.
If a PR is successful, no reply from the bot right now

Describe the solution you'd like

• If the PR of a contributor gets merged, there shall be a "Thank you for contributing" etc note (configurable by maintainer) from the bot

[issue-assigner]

Comment '@issue-assigner claim' to get this issue assigned or '@issue-assigner abandon' to get this issue unassigned.