Add auto backport functionality to backport PRs for releases

[naveentatikonda]

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.
  Related documentation: https://github.com/opensearch-project/OpenSearch/blob/main/DEVELOPER_GUIDE.md

• Add documentation for the same.

[VachaShah]

Hi @naveentatikonda thank you adding this! Can you also update this workflow and add an auto delete workflow as mentioned in https://github.com/opensearch-project/opensearch-plugins/blob/main/BACKPORT.md?

[naveentatikonda]

Hi @naveentatikonda thank you adding this! Can you also update this workflow and add an auto delete workflow as mentioned in https://github.com/opensearch-project/opensearch-plugins/blob/main/BACKPORT.md?

Sure, will add it.