Add new authentication system

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

@@ -0,0 +1,17 @@
+package emu.grasscutter.auth;
+
+import emu.grasscutter.server.http.objects.*;
+
+/**
+ * Handles username/password authentication from the client.
+ * @param <T> The response object type. Should be {@link LoginResultJson} or {@link ComboTokenResJson}
+ */
+public interface Authenticator<T> {
+    
+    /**
+     * Attempt to authenticate the client with the provided credentials.
+     * @param request The authentication request wrapped in a {@link AuthenticationSystem.AuthenticationRequest} object.
+     * @return The result of the login in an object.
+     */
+    T authenticate(AuthenticationSystem.AuthenticationRequest request);
+}