Support Application Default Credentials

[Stono]

Description of the feature request:

Please can you support ADC like the vast majority of the other google library.
Running locally, or on GKE, it's far better to use ADC than it is to generate tokens which require storage, injection and rotation.

What problem are you trying to solve with this feature?

Better security

Any other information you'd like to share?

No response

[demoncoder-crypto]

I worked on this issue, let me know if anything else is needed