Improve token verifier interface and tests

[mzaniolo]

Today we have two token verifiers, one for opaque tokens and one for jwts. We need to make a single verifier that can verify both tokens.

Since the opaque token verifier needs the private keys from an app, if the keys are not set the opaque token shouldn't be used.

This new interface for the token verifier should be tested and tests for the opaque token verifier should be also added since we are missing them