Implement proper handbook authentication (pt. 1)

src/main/java/emu/grasscutter/auth/HandbookAuthenticator.java

@@ -0,0 +1,34 @@
+package emu.grasscutter.auth;
+
+import emu.grasscutter.auth.AuthenticationSystem.AuthenticationRequest;
+import lombok.Builder;
+import lombok.Getter;
+
+/** Handles player authentication for the web GM handbook. */
+public interface HandbookAuthenticator {
+    @Getter @Builder
+    class Response {
+        private final int status;
+        private final String body;
+    }
+
+    /**
+     * Invoked when the user requests to authenticate.
+     * This should respond with a page that allows the user to authenticate.
+     *
+     * @route GET /handbook/authenticate
+     * @param request The authentication request.
+     */
+    void presentPage(AuthenticationRequest request);
+
+    /**
+     * Invoked when the user requests to authenticate.
+     * This is called when the user submits the authentication form.
+     * This should respond with HTML that sends a message to the GM Handbook.
+     * See the default handbook authentication page for an example.
+     *
+     * @param request The authentication request.
+     * @return The response to send to the client.
+     */
+    Response authenticate(AuthenticationRequest request);
+}