chore: Rework GitHub authentication flow

[vinokurig]

Signed-off-by: Igor Vinokur ivinokur@redhat.com

What does this PR do?

Exclude che-server requests from the GitHub authentication flow, instead read credentials file, mounted from the github-credentials-secret and extract the GitHub token.

Screenshot/screencast of this PR

What issues does this PR fix or reference?

fixes eclipse-che/che#20995
fixes eclipse-che/che#20788

How to test this PR?

1. Make sure that the GitHub is configured: https://www.eclipse.org/che/docs/next/administration-guide/configuring-authorization/#enabling-authentication-with-social-accounts-and-brokering_che
2. Create a factory with a private repo e.g. <che-url>/#https://github.com/vinokurig/test-extension/tree/devfile2 to authenticate GitHub for the user namespace.
3. Run Git: Clone and input a git url to a private repo e.g. https://github.com/vinokurig/test-extension.git:
   
4. Choose a folder to clone and click `Allow' button in the notification:
   

PR Checklist

As the author of this Pull Request I made sure that:

• The Eclipse Contributor Agreement is valid
• Code produced is complete
• Code builds without errors
• Tests are covering the bugfix
• The repository devfile is up to date and works
• Sections What issues does this PR fix or reference and How to test this PR completed
• Relevant user documentation updated
• Relevant contributing documentation updated
• CI/CD changes implemented, documented and communicated
• Optional Companion PR for updating the HappyPath tests is approved and ready to be merged

Reviewers

Reviewers, please comment how you tested the PR when approving it.

Happy Path Channel

HAPPY_PATH_CHANNEL=stable

[vinokurig]

depends on eclipse-che/che#21025

[codecov]

Codecov Report

Merging #1293 (e471754) into main (c299f59) will increase coverage by 3.50%.
The diff coverage is 42.43%.

@@            Coverage Diff             @@
##             main    #1293      +/-   ##
==========================================
+ Coverage   32.78%   36.29%   +3.50%     
==========================================
  Files         290      325      +35     
  Lines        9885    10600     +715     
  Branches     1457     1432      -25     
==========================================
+ Hits         3241     3847     +606     
- Misses       6641     6748     +107     
- Partials        3        5       +2     

Impacted Files	Coverage Δ
...theia-about/src/browser/about-che-theia-dialog.tsx	0.00% <0.00%> (ø)
...credentials/src/browser/che-credentials-service.ts	0.00% <0.00%> (ø)
...entials/src/browser/credentials-frontend-module.ts	0.00% <0.00%> (ø)
...eia-credentials/src/common/credentials-protocol.ts	0.00% <0.00%> (ø)
...eia-credentials/src/node/che-credentials-server.ts	0.00% <0.00%> (ø)
...s/src/node/che-theia-credentials-backend-module.ts	0.00% <0.00%> (ø)
...ashboard/src/browser/che-theia-dashboard-module.ts	0.00% <0.00%> (ø)
...ia-dashboard/src/browser/theia-dashboard-client.ts	0.00% <0.00%> (ø)
...rowser/src/browser/che-mini-browser-environment.ts	0.00% <0.00%> (ø)
...in-ext/src/browser/che-sidecar-file-system-main.ts	100.00% <ø> (ø)
... and 293 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9c6d63b...e471754. Read the comment docs.

[svor]

LGTM