[Feature request] Support github forks in github_pr_destination

Currently when using [github_pr_destination](https://github.com/google/copybara/blob/master/docs/reference.md#gitgithub_pr_destination), we are able to only raise PR by creating a branch in the destination repo.

destination/branch_name -> destination/main

This issue is a feature request to allow opening PRs from a branch in _Github fork_ of the destination instead of the destination itself.

destination_fork/branch_name -> destination/main