Add ability to override project id

[rogierknoester]

Description

Sometimes credentials are from a different GCP project than the service being used (e.g. a workload in project A needs to access apps in project B). These changes allow overriding the project id, as it is usually part of the service account's credentials already.

Fixes #62

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have made corresponding changes to the documentation

[jeromegamez]

I will release it tomorrow when I'm back at my desk!

[jeromegamez]

Done! https://github.com/kreait/firebase-bundle/releases/tag/5.4.0

[rogierknoester]

Thanks for the quick responses!