[ELY-2863] Add X509 client credential support

http/oidc/src/main/java/org/wildfly/security/http/oidc/Oidc.java

@@ -281,7 +281,8 @@ public String getValue() {
     public enum ClientCredentialsProviderType {
         SECRET("secret"),
         JWT("jwt"),
-        SECRET_JWT("secret-jwt")
+        SECRET_JWT("secret-jwt"),
+        X509_CERTIFICATE("x509-certificate")
         ;
 
         private final String value;

http/oidc/src/main/java/org/wildfly/security/http/oidc/X509CertificateCredentialsProvider.java

@@ -0,0 +1,54 @@
+/*
+ * JBoss, Home of Professional Open Source.
+ * Copyright 2021 Red Hat, Inc., and individual contributors
+ * as indicated by the @author tags.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+package org.wildfly.security.http.oidc;
+
+import static org.wildfly.security.http.oidc.Oidc.CLIENT_ID;
+
+import java.util.Map;
+
+import org.kohsuke.MetaInfServices;
+
+/**
+ * X509 certificate authentication. This assumes the actual certificate has been configured on the client SSLContext,
+ * which is done on the OIDC provider or secure-deployment.
+ * <br/>
+ * This class only serves as a placeholder to add the client_id argument to the body.
+ */
+@MetaInfServices(value = ClientCredentialsProvider.class)
+public class X509CertificateCredentialsProvider implements ClientCredentialsProvider {
+    @Override
+    public String getId() {
+        return Oidc.ClientCredentialsProviderType.X509_CERTIFICATE.getValue();
+    }
+
+    @Override
+    public void init(OidcClientConfiguration oidcClientConfiguration,
+                     Object credentialsConfig) {
+        // Not possible to check the key/truststore configuration from here. It needs access to the
+        // OidcJsonConfiguration object to check.
+    }
+
+    @Override
+    public void setClientCredentials(OidcClientConfiguration oidcClientConfiguration,
+                                     Map<String, String> requestHeaders,
+                                     Map<String, String> formParams) {
+        String clientId = oidcClientConfiguration.getResourceName();
+        formParams.put(CLIENT_ID, clientId);
+    }
+}