feat: Add ConflictException and mapper

Resolves #49 (closed)

This MR also adds default values to the oauth2 config fields in the DefaultDrupalTokenService. This allows applications that do not use this service to compile without the configs set.

core/src/main/java/org/eclipsefoundation/core/exception/ConflictException.java

+/*********************************************************************
+* Copyright (c) 2023 Eclipse Foundation.
+*
+* This program and the accompanying materials are made
+* available under the terms of the Eclipse Public License 2.0
+* which is available at https://www.eclipse.org/legal/epl-2.0/
+*
+* Author: Zachary Sabourin <zachary.sabourin@eclipse-foundation.org>
+*
+* SPDX-License-Identifier: EPL-2.0
+**********************************************************************/
+package org.eclipsefoundation.core.exception;
+
+import java.util.Collections;
+import java.util.Map;
+
+/**
+ * A custom exception used to indicate a conflict with the resource accessed by
+ * the client. Allows setting of header values upon creation.
+ */
+public class ConflictException extends RuntimeException {
+
+    private final Map<String, String> headers;
+
+    public ConflictException(String message) {
+        this(message, Collections.emptyMap());
+    }
+
+    public ConflictException(String message, Map<String, String> headers) {
+        super(message);
+        this.headers = headers;
+    }
+
+    public Map<String, String> getHeaders() {
+        return this.headers;
+    }
+}