Authorize GitHub requests with header instead of query parameter

[vinokurig]

What does this PR do?

Backport #924

Use GitHub Authorisation header in GitHub HTTP requests instead of token in query parameter.

Screenshot/screencast of this PR

What issues does this PR fix or reference?

eclipse-che/che#16425

How to test this PR?

1. Setup GitHub Identity provider
2. Start a workspace with GitHub Pull Requests plugin and a project cloned from GitHub by https url.
   You can use this devfile to start a workspace:

apiVersion: 1.0.0
metadata:
  name: wksp-custom-dwdiu
projects:
  - name: che-theia
    source:
      location: 'https://github.com/eclipse/che-theia.git'
      type: git
      branch: master
components:
  - type: chePlugin
    reference: 'https://raw.githubusercontent.com/eclipse/che-plugin-registry/master/v3/plugins/ms-vscode/vscode-github-pullrequest/0.20.0/meta.yaml'
  - type: cheEditor
    reference: 'https://raw.githubusercontent.com/chepullreq4/pr-check-files/master/che-theia/pr-939/simple/che-theia-editor.yaml'

3. Sign-in to the GitHub Pull Requests plugin.

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

Reviewers

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

Happy Path Channel

HAPPY_PATH_CHANNEL=stable

[benoitf]

backporting LGTM

[azatsarynnyy]

I'm approving as it's a backport from the master branch.