Refactor mTLS certificate loading logic into google-auth-library

[bcoe]

@bshaffer thinks that it would be better if we had our certificate loading helper in google-auth-library-nodejs, rather than in gax.

I like the idea for this refactor, because it would make it easier to reuse this certificate loading logic in REST libraries.

This doesn't change how we integrate the generated code with mTLS, so I think we can take this on as an eventual improvement.