Add support for rm_file in GitHubFileSystem implementation

[mecit-san]

This PR adds support for the rm_file method in the GitHubFileSystem implementation of fsspec, enabling file deletion using the GitHub REST API.

The GitHub API for managing file contents requires:

• A commit message for each action (add/update/delete), similar to editing files via the GitHub web interface.
• The SHA of the target file’s latest version when deleting or updating, to avoid race conditions.

This implementation adheres to those requirements, making rm_file work reliably for GitHub-backed filesystems.

Note: This PR focuses solely on file deletion. I plan to follow up with a separate PR to implement the remaining methods and functionality (e.g., adding file, updating file, etc.) for full GitHubFileSystem support.

#1836