Add client certificate support

[itzikbekelmicrosoft]

• Added functionality to support Azure AD application certificate authentication.
• Updated documentation to include steps for setting up certificate authentication.
• Ensured that client secret authentication remains the default for existing setups to avoid breaking changes.

Testing done

• Local Testing: Successfully tested the new certificate authentication feature on a local Jenkins instance.
• Azure VM Testing: Deployed and tested on a Jenkins instance container running in an Azure VM to ensure compatibility and functionality.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[itzikbekelmicrosoft]

@timja Please review

[timja]

I've tested that client secret still works and made some minor UI tweaks.

I haven't tested the client secret functionality but code looks good

[itzikbekelmicrosoft]

I've tested that client secret still works and made some minor UI tweaks.

I haven't tested the client secret functionality but code looks good

You meant you haven't tested the client certificate right? I have tested it with two different certificates and it functions as expected.

[timja]

Really close, I think the tests could be improved a little bit though

[KalleOlaviNiemitalo]

There are some whitespace issues: removal of final newlines in files, varying numbers of blank lines between methods, extra indentation in one method. Not a big deal though.

[timja]

Thanks for the contribution!