Add OAuth in AuthenticationSystem

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

@@ -17,6 +17,7 @@ public final class DefaultAuthentication implements AuthenticationSystem {
     private final Authenticator<LoginResultJson> tokenAuthenticator = new TokenAuthenticator();
     private final Authenticator<ComboTokenResJson> sessionKeyAuthenticator = new SessionKeyAuthenticator();
     private final ExternalAuthenticator externalAuthenticator = new ExternalAuthentication();
+    private final OAuthAuthenticator oAuthAuthenticator = new OAuthAuthentication();
     
     @Override
     public void createAccount(String username, String password) {
@@ -53,4 +54,9 @@ public final class DefaultAuthentication implements AuthenticationSystem {
     public ExternalAuthenticator getExternalAuthenticator() {
         return this.externalAuthenticator;
     }
+
+    @Override
+    public OAuthAuthenticator getOAuthAuthenticator() {
+        return this.oAuthAuthenticator;
+    }
 }