BE: Auth: Support GCP IAM Auth

[Muchembi]

• Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview)
resolves #1012

Is there anything you'd like reviewers to focus on?
The front end especially.

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• No need to
• Manually - Built an image and used it in a k8s helm deployment. I was able to authenticate to my GMSK and interact with the cluster as normal.
• Unit checks
• Integration checks
• Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

Check out Contributing and Code of Conduct

A picture of a cute animal (not mandatory but encouraged)

[github-actions]

Hi Muchembi! 👋

Welcome, and thank you for opening your first PR in the repo!

Please wait for triaging by our maintainers.

Please take a look at our contributing guide.

[Haarolean]

Thanks for your PR. Can we also get a test like AzureEntraLoginCallbackHandlerTest but for GCP? Feel free to mock it heavily.

[Haarolean]

@Muchembi also, could you please update our docs to reflect new changes?
https://github.com/kafbat/ui-docs