Fix github authentication built-in for electron case

[tsmaeder]

What it does

Add dummy command to fix github authentication built-int flows. Also includes code to consider sessions which are not created, but restored from storage at registration time.

When deploying Theia as a web-app, one would normally use a web application flow (https://docs.github.com/en/apps/oauth-apps/building-oauth-apps/authorizing-oauth-apps#web-application-flow). However, this will not work, because the VSCode extension explicitly restricts the domains for which that flow can be used. Also, we obviously do not know the "client secret" of the VS Code Github ouath app.

Fixes #13599
Partial fix for #12821

Contributed on behalof of STMicroelectronics

How to test

1. Install the github-authentication and gitlens plugins
2. The gitlens plugin will ask for "workspace trust": say "yes"
3. Click the little manikin icon on the bottom of the left side bar.
4. Observe if you are logged into github, the menu will display a menu allowing you to "sign out"
5. If logged in, sign out, for some reason, you'll have to restart Theia now
6. Open a file that is under git control
7. Observe: gitlens will display text after the end of the current line. If you hover, you'll get a link "Connect to Github"
8. Click that link
9. Follow the instructions to sign into github
10. Observe: when you now hover over the gitlens text, you'll get a link to the relevant PR for the line, instead of "Connect to Github"

Follow-ups

Make "github" authentication work for the web app case.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[martin-fleck-at]

Thank you very much, Thomas! I can confirm the the workflow now works (even though it seems needlessly complicated but that is a different issue entirely ;-) ) with the Electron app and (still) fails with the Browser app. So definitely an improvement.