[Resolver]: Parameter for merge branch in Github action

[malhotra5]

What problem or use case are you trying to solve? Any way to specify resolver to branch / merge off of dev instead of main?

Describe the UX of the solution you'd like

Do you have thoughts on the technical implementation? Add a target_branch parameter to openhands/resolver/examples/openhands-resolver.yml and .github/workflows/openhands-resolver.yml; pass this to send_pull_request in the workflow

The parameter should be stored in a repo variable.

Update docs/modules/usage/how-to/github-action.md with details on parameter.

Describe alternatives you've considered

Additional context