cli: Allow Application Default Credentials discovery for GCP

[xytis]

When deploying tusd under GCP, it is better to use integrated Application Default Credentials discovery mechanism, that exists under most google SDK's.

This change simply allows not providing any string as credential file, and then client library defaults to trying ADC as per https://cloud.google.com/docs/authentication/application-default-credentials

If this proposal is acceptable, I could update the readme as well.

[Acconut]

The idea looks good to me, thank you for bringing this up.

What error does the SDK and thus tusd now return if no GCS_SERVICE_ACCOUNT_FILE was given and ADC failed as well? We must ensure that this still gives a descriptive error message.

I could update the readme as well.

That would be great!

[xytis]

Default error is okey-ish:

2024/10/22 07:10:01.750054 Attempting to use default GCS credentials because no GCS_SERVICE_ACCOUNT_FILE env var was provided
2024/10/22 07:10:01.802682 Unable to create Google Cloud Storage service: dialing: google: could not find default credentials. See https://cloud.google.com/docs/authentication/external/set-up-adc for more information

[Acconut]

Yes, that seems acceptable, especially if our documentation covers authentication.

[xytis]

Ok, updated the docs, plus I removed the warning message (since it now is explained in the docs).

[Acconut]

Thank you for the contribution!