feat: add debug logging support

[feywind]

Adds debug logging support for auth. This needs a bit more work/testing still.

It's also waiting on this to finish:
googleapis/gax-nodejs#1669

[feywind]

I need to update this for when the debug logger piece is officially reviewed and released, and there's a system test thing to look at, but the code itself is largely ready, I think.

[d-goog]

I think it may be cleaner and flexible to add this to the base AuthClient class and have it as a member or static method.

This will allow:

• Customers to change/modify the log functionality (if they want)
• Dedupe the import statements across the files

Additionally, in the next major we're planning to unify the Transporter instance into AuthClient via this PR:

• https://github.com/googleapis/google-auth-library-nodejs/pull/1860/files#diff-1be86cf1a764f43a299d2d037e8db93a74904846592cfe5f31c0981079c08992R256

It would be an even smaller migration after that has landed as we could extend the GaxiosOptions interface with a Symbol (which would note the type of log) and use log.info on every request (as all requests would use that _request method).

[feywind]

Thanks for the quick review. I'll make that change tomorrow.

[feywind]

A lot of the logging can be done through AuthClient now, good call. There are a few objects that don't seem to derive ultimately from AuthClient, so I moved makeLog() into the class itself to make it cleaner. (makeLog caches the loggers by name, so there's little harm in calling it multiple times.)