feat(multiple-auth): add support for multiple auth

src/main/java/io/apimatic/coreinterfaces/authentication/Authentication.java

@@ -5,17 +5,39 @@
 /**
  * To setup methods for authentication
  */
-public interface Authentication {
+public abstract class Authentication {
 
     /**
-     * Apply the authentication on the httpRequest
-     * @param httpRequest the request on which authentication is being applied
-     * @return the authenticated request
+     * Stores the error message for the auth scheme.
      */
-    Request apply(Request httpRequest);
+    private String errorMessage;
 
     /**
-     * Validates the auth params for the httpRequest
+     * Applies the authentication on the httpRequest.
+     * @param httpRequest the request on which authentication is being applied.
+     * @return the authenticated request.
      */
-    void validate();
+    public abstract Request apply(Request httpRequest);
+
+    /**
+     * Validates the credentials for authentication.
+     * @return true if the auth credentials are valid, false otherwise.
+     */
+    public abstract boolean validate();
+
+    /**
+     * Returns the error message if the auth credentials are not valid.
+     * @return The string message whenever the auth credentials are not valid.
+     */
+    public String getErrorMessage() {
+        return errorMessage;
+    }
+
+    /**
+     * Sets the error message for invalid auth credentials.
+     * @param errorMessage the error message to set.
+     */
+    public void setErrorMessage(String errorMessage) {
+        this.errorMessage = errorMessage;
+    }
 }