PKCE for GitHub

**Is your feature request related to a problem? Please describe.**
Currently in order to use GitHub as a backend, you need to run a separate server to facilitate the authentication: https://www.netlifycms.org/docs/github-backend/

Running a separate server is not required for GitLab because it supports PKCE: https://www.netlifycms.org/docs/gitlab-backend/

It would be great if we could do the same for GitHub, so I'm trying to gain attention and drum up support for GitHub to implement this.

**Describe the solution you'd like**
I've opened a discussion on GitHub, but it would help to get more community support. Please consider up-voting, liking, and commenting on the thread if this is something you'd like to see as well: https://github.com/orgs/community/discussions/15752

**Describe alternatives you've considered**
The alternative is to run your own [external oauth client](https://www.netlifycms.org/docs/external-oauth-clients/), but one of the main advantages of running a Jamstack site with a git-backed CMS is not having to run a server, so IMO this defeats the purpose.

**Additional context**
Other projects that this could be relevant for are [TinaCMS](https://github.com/tinacms/tinacms) and my project [Plenti](https://github.com/plentico/plenti)