Disable rpc.enhancedClasses by default at runtime

[niloc132]

Logs warnings at compile time, indicating which classes need to be cleaned up to remove this feature.

Mitigation for #9709

[niloc132]

The good news is that all tests passed with this change https://github.com/niloc132/gwt/actions/runs/7267798597

The bad news is that this means that the rpc.enhancedClasses feature apparently has no tests - probably only bad news in retrospect, given that we are trying to turn it off.

[niloc132]

In light of #9709 (comment), it might be wise to generalize these error messages slightly, and if the developer doesn't believe that JPA/JDO is in use, to just remove the annotations and see if that still works.

[niloc132]

Testing to confirm the new behavior:

1. Started with the DynaTable example, confirmed it started and worked correctly
2. Added <extend-configuration-property name="rpc.enhancedClasses" value="com.google.gwt.sample.dynatable.client.Person" /> to the .gwt.xml file
3. Confirmed that the compiler logged a warning:

   [java]             Rebinding com.google.gwt.sample.dynatable.client.SchoolCalendarService
   [java]                Invoking generator com.google.gwt.user.rebind.rpc.ServiceInterfaceProxyGenerator
   [java]                   Generating client proxy for remote service interface 'com.google.gwt.sample.dynatable.client.SchoolCalendarService'
   [java]                      [WARN] The class com.google.gwt.sample.dynatable.client.Person has JPA/JDO annotations or is explicitly configured as an enhanced class using the configuration property rpc.enhancedClasses. This makes the server vulnerable to an issue with deserialization of unsafe data. See https://github.com/gwtproject/gwt/issues/9709 for more information.
   

4. Confirmed that the server would not load the policy and logged a server error:

   [java] 2023-12-23 12:24:51.073:INFO:oejshC.ROOT:qtp559361838-84: calendar: ERROR: Service deserializes enhanced JPA/JDO classes, which is unsafe. Review build logs to see which classes are affected, or set gwt.enhancedClasses.enabled to true to allow using this service. See https://github.com/gwtproject/gwt/issues/9709 for more detail.
   [java] 2023-12-23 12:24:51.074:INFO:oejshC.ROOT:qtp559361838-84: calendar: WARNING: Failed to get the SerializationPolicy 'D08326A5FC5842E352DF103522C617ED' for module 'http://127.0.0.1:8888/dynatable/'; a legacy, 1.3.3 compatible, serialization policy will be used.  You may experience SerializationExceptions as a result.
   

5. Note also that the client could not load data correctly with the legacy policy, and had its own error.
6. Added -Dgwt.enhancedClasses.enabled=true to the JVM args, confirmed that the above error was replaced by a warning and the application worked:

   [java] 2023-12-23 12:38:39.032:INFO:oejshC.ROOT:qtp448000133-82: calendar: WARNING: Service deserializes enhanced JPA/JDO classes, which is unsafe. See https://github.com/gwtproject/gwt/issues/9709 for more detail on the vulnerability that this presents.
   

7. Next, removed the configuration property, and instead added @javax.persistence.Entity to the Person bean, org.datanucleus:javax.persistence:2.2.3 added to war/WEB-INF/lib/, and repeated the test from step 3.