Support adding static known issuer keys

Overview

This MR adds support for specifying public keys of the known issuers. For example, a service doesn't implement OIDC endpoints, but we still want to include its public key as an accepted one.

This solution is the closest to OIDC, so when the expected service implements OIDC endpoints, its endpoints are specified into GLGO_KNOWN_ISSUERS and the keys are removed from GLGO_KNOWN_ISSUER_KEYS