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

TODO.md

@@ -1,5 +1,8 @@
 # TODO
 
+- [ ] better OIDC support
+- [ ] better support of `profile` `openid` `email` `roles` scopes
+
 - [ ] i18n strings in the http website.
 
 - [ ] Instance customization support
@@ -50,3 +53,5 @@
 - [X] basic docker setup
 - [ ] make `docker stop` working (handle SIGTERM/SIGINT)
 - [ ] implement docker secrets. https://docs.docker.com/engine/swarm/secrets/
+
+- [ ] Find a minimal OpenID client implementation like Listmonk but a little bit more mature