Today, we don't have a clean way to change AuthClient scopes uniformly between instances, and different AuthClients offer varying functionality like this:

https://github.com/googleapis/google-auth-library-nodejs/blob/6c0a6bdc156c1098b09e85e82cabadaa73e7367f/src/auth/jwtclient.ts#L100-L115

We should offer a uniform API for changing scopes for all AuthClients. Additionally, once this is implemented GoogleAuth#scopes should update its cachedCredentials's scopes (perhaps via a simple getter/setter hook on GoogleAuth itself).

Dependencies:

• feat: Unify Base AuthClient Options google-auth-library-nodejs#1663
• feat!: Normalize #scopes args and properties to string[] #518