Add ability to break cache if RS256 ID token kid is not found

[joshcanhelp]

Changes

Add WP_Auth0_JwksFetcher->getKey() for use in WP_Auth0_AsymmetricVerifier to fetch a new JWKS when an incoming token KID is not found in the cached set. This library doesn't accept arbitrary requests that involve token validation and the endpoint that validates tokens is protected by a state parameter so there is not much of a risk of invalid kid spamming.

References

SDK-1224

Testing

• This change adds unit test coverage
• This change has been tested on WP 5.3.2 and PHP 7.1

Checklist

• All existing and new tests complete without errors
• All code quality tools/guidelines in the Contribution guide have been run/followed
• All active GitHub CI checks have passed