Added a couple of new internaloptions, WithDefaultAudience and
WithDefaultScopes, which will be used to start using self-signed
JWTs by default when certain criteria are met:

1. Authenticating with a service account.
2. User does not pass explicitly provide their own scopes.
3. An audience is provided.

In the future gapic client will begin to pass these new
internaloptions which will enable them to authenticate with with
a JWT signed by a service account. This is a non-standard oAuth2
flow, and is an optimization to save an extra network request.