Load all OIDC RPs by default again

In 2.17 I added lazy-loading capabilities to the OIDC issuer (great) and decided not to load any RPs by default anymore, and rely on the new loading system even for configuration-provided RPs (not so great, it caused many regressions)

In 2.18, I think we should go back to loading all conf-provided RPs to make sure not too many things will break

This MR will eagerly load all RPs defined in conf during OIDC issuer init

@clement_oudot can you check that it fixes #3058 (closed) ?