feat: add impersonated credential handler

[mvaldi]

Hi Firebase Team!

Currently Firebase SDK cannot handle impersonated credentials, it happens that I need to impersonalize the service account to be able to access the Firestore of another project id, which, for me, the impersonated of the service account is necessary, since in the company we have disabled the json keys for security reasons

So this PR has the solution to be used with impersonated credentials without errors.

I also leave an example in docstring how to use