Validate the subject in an oidc ping against a list of logged in subjects.
This resolves the issue that multiple connected FRP clients with different
OIDC clients result in a failing ping. The ping would fail because the
subject in memory would be the value of the last logged in FRPC.
This change also changes the constructor of OidcAuthVerifier to take
a TokenVerifier interface. This will not change production behavior, but makes
testing easier because we can inject a mock verifier during testing.
Resolves: #4466
