Provide automatic git authentication to GitHub

[joaomoreno]

Fixes #96241

The Git extension currently already hijacks into the GIT_ASKPASS mechanism to provide UI to prompt the user for credentials when they try to run git commands over an HTTPS-based remote and do not have any credential manager installed:

This PR adds a new registerCredentialsProvider method to the Git extension API which allows other extensions to contribute to that mechanism and provide credentials themselves, before the last resort of prompting the user is used. Using the new vscode.authentication API, an extension can easily provide GitHub access tokens as credentials. We would have automatic authentication for all Git: Pull, Git: Push, etc. commands in our UI for GitHub repositories!

We could implement this in the GHPR extension, but another option is to simply implement this in the github-authentication core extension. Why not have this built into the core?

This PR implements exactly that.

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The authentication authorization dialog currently looks like this:

It feels strange that GitHub Authentication appears in there.

• Should we simply rename the extension to GitHub?
• Or even rename the extension's id to github, for that matter?
• Also, should this prompt even show up for built-in extensions at all?

Also, while this tackles git commands in the UI, it doesn't address terminal commands yet. Once #96061 lands, we'll be able to simply add a GIT_ASKPASS env var to terminals and even git pull, git push, etc. commands in the integrated terminal would have GitHub automatic authentication working.

cc @egamma @kieferrm @RMacfarlane @Tyriar @chrisdias

[Tyriar]

Should we simply rename the extension to GitHub?

I was always hoping that eventually the GHPR extension would be simply "GitHub", especially now since it has issues in it as well.

[joaomoreno]

@RMacfarlane had a great idea to tackle the dialog without any big decisions: we can make it such that the Git extension requests access to the creds, making it read:

The extension 'Git' wants to sign in using GitHub.

Which I really like. 👍

[joaomoreno]

Alright, ended up moving the code into the Git extension for now, which means the dialog will look nice.