[Auth] feat: Missing full payload in identity token from GCECredentials

**Is your feature request related to a problem? Please describe.**
We are missing the email of the authorized party when we decode the identity token on Cloud Run in PHP.

At this moment the PHP implementation of the GCECredential class is missing the full payload param on the identity token metadata server request. This is already in place in other SDK's like the python SDK:

https://github.com/googleapis/google-auth-library-python/blob/9cd67425e95faab15e57b258a70506b02bccb799/google/auth/compute_engine/credentials.py#L391

**Describe the solution you'd like**
My suggestion would be to add the param `format=full` for requests going to `v1/instance/service-accounts/default/identity`