feat: implement quota project from env var in google/auth

[bshaffer]

No description provided.

[bshaffer]

reopened at the request of @sai-sunder-s

[vishwarajanand]

It appears that if GOOGLE_CLOUD_QUOTA_PROJECT is set then setting $quotaProject shouldn't be required for service clients. Questions:

1. We can't remove them from clients since they might be breaking? So, which one will be preferred?
2. In case both are passed with different values, should we add a doc block in clients to clarify this?

[bshaffer]

It appears that if GOOGLE_CLOUD_QUOTA_PROJECT is set then setting $quotaProject shouldn't be required for service clients.

I do not think setting quotaProject is required anywhere.

1. We can't remove them from clients since they might be breaking? So, which one will be preferred?

Removing it would definitely be breaking. Also we don't want to remove it. There is no "preferred", it's just that in some situations a user might want to use an env var, and in some situations a parameter.

2. In case both are passed with different values, should we add a doc block in clients to clarify this?

In the case that both are set, the parameter is used. I'm all for adding clarity on this, and will add documentation to reflect this.

[vishwarajanand]

Lgtm