Supporting service account key format OR user credential formats

[msdrigg]

This PR supports parsing both formats in either the GOOGLE_APPLICATION_CREDENTIALS env variable or the ~/.config/gcloud/application_default_credentials.json file.

This is part 1 of the PRs addressing #56

For context see https://github.com/hrvolapeter/gcp_auth/pull/75#issuecomment-1683894661

[msdrigg]

@djc I need to do real world testing on this part, but is this what you're looking for as of part 1?

[msdrigg]

Tested this PR with real world keys

• Tested with GOOGLE_APPLICATION_CREDENTIALS and ~/.config/gcloud/application_default_credentials.json and verified the new tagged enum is used in both cases
• Tested with service account key and user creds, and verified that I can make API calls

[msdrigg]

This should be ready for final checks at this point

[djc]

This is on my list to review, but it's been busy. Hope to get to it soon.

[msdrigg]

No rush. Just wanted to make sure it wasn't waiting on me.