feat: add support for specifying a clientAuthenticationMethod for OIDC

Author: dervoeti

crates/stackable-operator/src/crd/authentication/oidc/mod.rs

@@ -90,6 +90,40 @@ pub mod versioned {
         Keycloak,
     }
 
+    /// OAuth2 client authentication methods as defined in the OpenID Connect Core spec.
+    /// These methods are used by clients to authenticate to the authorization server
+    /// when using the token endpoint.
+    ///
+    /// See <https://openid.net/specs/openid-connect-core-1_0.html#ClientAuthentication> for details.
+    #[derive(
+        Clone, Copy, Debug, Default, Deserialize, Eq, Hash, JsonSchema, Ord, PartialEq, PartialOrd,
+        Serialize,
+    )]
+    #[serde(rename_all = "snake_case")]
+    pub enum ClientAuthenticationMethod {
+        /// Authenticate using HTTP Basic authentication with client_id and client_secret.
+        /// This is the default method according to the OIDC spec.
+        #[default]
+        #[serde(rename = "client_secret_basic")]
+        ClientSecretBasic,
+
+        /// Send client_id and client_secret in the request body.
+        #[serde(rename = "client_secret_post")]
+        ClientSecretPost,
+
+        /// Authenticate using a JWT signed with an HMAC SHA algorithm using the client_secret.
+        #[serde(rename = "client_secret_jwt")]
+        ClientSecretJwt,
+
+        /// Authenticate using a JWT signed with the client's private key.
+        #[serde(rename = "private_key_jwt")]
+        PrivateKeyJwt,
+
+        /// No client authentication (for public clients or implicit flow).
+        #[serde(rename = "none")]
+        None,
+    }
+
     /// OIDC specific config options. These are set on the product config level.
     #[derive(
         Clone, Debug, Deserialize, Eq, Hash, JsonSchema, Ord, PartialEq, PartialOrd, Serialize,
@@ -111,6 +145,25 @@ pub mod versioned {
         #[serde(default)]
         pub extra_scopes: Vec<String>,
 
+        /// The OAuth2 client authentication method to use for token endpoint requests.
+        /// Defaults to [`ClientAuthenticationMethod::ClientSecretBasic`].
+        ///
+        /// The contents and format of the `clientCredentialsSecret` are dependent on the selected
+        /// method. For example, [`ClientAuthenticationMethod::ClientSecretBasic`] and
+        /// [`ClientAuthenticationMethod::ClientSecretPost`] require a client secret string, whereas
+        /// [`ClientAuthenticationMethod::PrivateKeyJwt`] requires a private key.
+        ///
+        /// See [`ClientAuthenticationMethod`] for available options.
+        #[schemars(
+            description = "The client authentication method used when communicating with the token \
+            endpoint. Defaults to `client_secret_basic`. The required contents of \
+            `clientCredentialsSecret` depend on the chosen method: secret-based methods \
+            (`client_secret_basic`, `client_secret_post`, `client_secret_jwt`) expect a client \
+            secret, while `private_key_jwt` expects a private key."
+        )]
+        #[serde(default)]
+        pub client_authentication_method: ClientAuthenticationMethod,
+
         // If desired, operators can add custom fields that are only needed for this specific product.
         // They need to create a struct holding them and pass that as `T`.
         #[serde(flatten)]