feat: add useJWTAccessAlways and defaultServicePath variable

[sofisl]

Piecemeal version of #1196

[bcoe]

There are two places I notice where we set defaultScopes but don't set client.useJWTAccessAlways = this.useJWTAccessAlways;:

google-auth-library-nodejs/src/auth/googleauth.ts

Line 270 in 57e4dad

credential.defaultScopes = this.defaultScopes;

and

google-auth-library-nodejs/src/auth/googleauth.ts

Line 286 in 57e4dad

credential.defaultScopes = this.defaultScopes;

I was also concerned by:

https://github.com/googleapis/google-auth-library-nodejs/pull/1204/files#diff-535a540500eec341838bd88c84b1795313a8ff9b556bad87ef89fb2fca061630R573

Where I notice you set useJWTAccessAlways, but we miss populating defaultScopes. Should we be setting defaultScopes here, or I wonder did we already set it in a related code path so we don't need to? we should probably double check.

To help protect future generations from having hiccups like these, why don't we add a private helper:

setAdditionalJWTConfig or something along those lines, which always sets all three variables?

[bcoe]

I think this is a special edge case where we don't need to set useJWTAccessAlways, where we're providing an explicit API key to use (which I'm betting is why we don't set defaultServicePath either).