Implement OIDC token exchange

This feature exists in Keycloak. Here is a simple implementation: just to declare the list of Relying Party allowed to query an access_token for the current Relying Party.

This implementation is split into:

• a base class usable for all oidcGotTokenExchange plugins (used also by Matrix token exchange)
• the OIDC Internal Token Exchange itself

Related to #3148 (closed)