Fix handling of JWT without key ID

When a key ID (kid) is not included in the JWT, that means we don't know anything about which signing key to use. The json-jwt library expects the kid value to be present if a JWK Set is presented. If a missing kid exception is raised, we now iterate through each key to find one that works.

Closes https://github.com/m0n9oose/omniauth_openid_connect/issues/64

Relates to gitlab-org/gitlab!34030 (merged)