feat: support OIDC id_token

- generate JWT id_token in token exchange
- store optional nonce in authorization object
- switch to RS256 algorithm for JWT signature
- add JWKs endpoint to provide OIDC clients with public keys

tests/hurl_integration/user_invitation_scenario/config.toml

@@ -1,3 +1,4 @@
+signing_key = "tmp/secrets/signing.key"
 applications = []
 
 roles = []