Feature request: Allow workload IAM as an auth source

[apex-omontgomery]

This plugin currently supports default credentials and explicitly defined credential file as specified in the configuration file.

For our use case, we prefer the ability to specify and use workload iam. When we use this it adds simplicity to our systems, and per docs:

recommended way for your Google Kubernetes Engine (GKE) applications to consume services provided by Google APIs.

I understand this is not a feature request that would be entertained for the rest of the stack as they wish to manage auth internally (and supportability for every type of auth is a concern). As this is a google specific plugin I would be pleased if that policy doesn't apply here.