Add auto-approve action for Coder Desktop releases

[ThomasK33]

The Coder Desktop repository is configured so that every push to the main branch or merge of a pull request triggers a build of the application and uploads it to GitHub. This setup simplifies the process of testing and sharing pre-release versions, eliminating the need to compile the application locally and follow multiple steps.

We need to update the coder-desktop-preview cask file to reflect a newer version; otherwise, Homebrew will not recognize that an update is available. To streamline this process, we have implemented automation on the Coder Desktop side that regenerates the cask file with each new version/tag. However, this can become cumbersome, as each merged pull request (PR) on Coder Desktop results in one PR in this repository with only a minor change.

We should create a GitHub Action that automatically approves and enables 'merge when ready' for pull requests made by the cdrci user, specifically on a particular branch or with a specific label.
The action needs to be generic and not specific to Coder Desktop so that it can be easily extended to accommodate other formulas and casks.