Implement OIDC token exchange

Following #2838 (closed), the idea here is to implement a "standard" token exchange following Keycloak implementation.

Proposed implementation

• a base class that does the job except for validation for which it calls validate($req, $rp, $subjectToken, $requestedTokenType) from the sub class
• sub-classes:
  • internal token exchange
  • Matrix token exchange
  • Google token exchange
  • Facebook token exchange
  • ...