Add GitHub Action to support /assign and /unassign comment commands

[yugalkaushik]

Fixes #3551

This PR introduces a GitHub Action that allows contributors to assign or unassign themselves from issues and pull requests using simple comment commands.

Changes:

• /assign: Assigns the commenter to the issue or PR.
• /unassign: Removes the commenter as an assignee.

I have verified that this pull request:

• [x] has no linting errors (npm run lint)
• [x] has no test errors (npm run test)
• [x] is from a uniquely-named feature branch and is up to date with the develop branch.
• [x] is descriptively named and links to an issue number, i.e. Fixes #123