fix: expand on x-goog-user-project to handle auth.getClient()

[bcoe]

What is this?

This expands on the initial implementation of x-goog-user-project header population, such that headers are appropriately populated when auth.getClient().getRequestHeaders() is called, the initial approach only supported auth.getRequestHeaders().

Thanks to @JustinBeckwith for pointing out this code-path which would have bitten us.

Implementation Details

This implementation is closer to @broady's initial recommendation of having the logic in UserRefreshClient rather than my suggestion of GoogleAuth, however, I've moved the logic for header population into OAuth2Client, this approach seemed elegant because:

• OAuth2Client already implements getRequestHeaders (and several clients, like UserRefreshClient rely on its implementation.
• It puts us in a position to support quota_project across clients.
• It seemed the most elegant of a few approaches I tried.

[broady]

LGTM