[Feature request] Pushing files to repos

It would be nice to be able to create or update a file on Github through the API, as GitHub.jl does.

I'd be willing to work on this at some point, but right now I'm a bit swamped.

Start by comparing GitForge.jl's existing API surface with the file create/update capability described for GitHub.jl. Determine the appropriate repository file entry point and the behavior required for both creating and updating a file through a Git forge API. Done means the feature is implemented with coverage for both operations.

Written by the indexing model from the issue text.