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

lib/kernel/src/consts.rs

@@ -1,4 +1,5 @@
 pub const DEFAULT_DB_PATH: &str = "/var/lib/minauthator/minauthator.db";
 pub const DEFAULT_ASSETS_PATH: &str = "/usr/local/lib/minauthator/assets";
 pub const DEFAULT_CONFIG_PATH: &str = "/etc/minauthator/config.toml";
+pub const DEFAULT_SIGNING_KEY_PATH: &str = "/etc/minauthator/secrets/jwt.key.pem";