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

[davidcui1225]

Closing as PR #445 addresses this issue