Add auto backport functionality to backport PRs for releases

[VachaShah]

Coming from meta issue opensearch-project/opensearch-plugins#108:

• [ ] Add auto-backport PR creation Github action to make sure all the PRs are backported for a particular release.
• [ ] Add auto-delete for merged auto-backport PRs to clean up the branches created during auto-backport.
• [ ] Add documentation for the same.

Related documentation: https://github.com/opensearch-project/opensearch-plugins/blob/main/BACKPORT.md

[kkhatua]

Perftop will be on a deprecation path while we work towards creating a Dashboard-based equivalent.