Add auto backport functionality to backport PRs for releases

[VachaShah]

In order to avoid creating backport PRs for each release and miss some of them in the process, we can add a Github action backport.yml that would create automatic backport PRs when the original PRs are labeled accordingly.

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

Creating this meta issue to track this feature for clients.

• opensearch-go opensearch-project/opensearch-go#100
• opensearch-dsl-py opensearch-project/opensearch-dsl-py#44
• opensearch-java opensearch-project/opensearch-java#142
• opensearch-js opensearch-project/opensearch-js#218
• opensearch-net opensearch-project/opensearch-net#37
• opensearch-php opensearch-project/opensearch-php#49
• opensearch-py opensearch-project/opensearch-py#148
• opensearch-ruby opensearch-project/opensearch-ruby#58
• opensearch-rs opensearch-project/opensearch-rs#41

[VachaShah]

Closing this issue since all the mentioned clients have the required changes.