Enable use of 'Authenticate to Google Cloud' action #299

[seth-acuitymd]

Reimplementing https://github.com/FirebaseExtended/action-hosting-deploy/pull/299 -

Makes the firebaseServiceAccount optional so that GOOGLE_APPLICATION_CREDENTIALS doesn't get overwritten if the option is not defined. Updated the readme examples to use 'Authenticate to Google Cloud' action with workload identity federation.

I still need to confirm this works as intended -

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