Use builtin google.auth utility to load credentials

[brimoor]

Use google.auth.load_credentials_from_file() to load GCP credentials.

Previously we only supported service accounts, now we support all things that Google supports:

The credentials file must be a service account key, stored authorized user credentials or external account credentials.

[swheaton]

Lgtm; what cred types did you test with

[ehofesmann]

Lgtm; what cred types did you test with

This PR worked for service account creds. I still haven't figured out a way to generate WIF external credentials

[brimoor]

I confirmed that both of these work:

import eta.core.serial as etas
import google.auth as ga

# Service account

credentials_path = "/path/to/gcp-service-account.json"
credentials, _ = ga.load_credentials_from_file(credentials_path)

# External account

credentials_path = "/tmp/gcp-credentials.json"
d = {
  "type": "external_account",
  "audience": "//iam.googleapis.com/projects/${OpenShift_GCP_PROJECT_NUMBER}/locations/global/workloadIdentityPools/${OpenShift_WORKLOAD_IDENTITY_POOL}/providers/${OpenShift_WORKLOAD_IDENTITY_POOL}",
  "subject_token_type": "urn:ietf:params:oauth:token-type:jwt",
  "token_url": "https://sts.googleapis.com/v1/token",
  "credential_source": {
    "file": "/var/run/secrets/openshift/serviceaccount/token",
    "format": {
      "type": "text"
    }
  },
  "service_account_impersonation_url": "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/${GCP_SA}@${GCP_PROJECT_ID}.iam.gserviceaccount.com:generateAccessToken"
}

etas.write_json(d, credentials_path)
credentials, _ = ga.load_credentials_from_file(credentials_path)