Harden XML parsing, serialization, and randomness (#3075)

This commit introduces several security hardening improvements across the codebase:
1. XML Processing: Hardened `TransformerFactory` and `SchemaFactory` instantiations in `EppMessage.java` by explicitly enabling `XMLConstants.FEATURE_SECURE_PROCESSING` and disabling external schema access.
2. Randomness: Replaced instances of `java.util.Random` with `java.security.SecureRandom` in `SelfSignedCaCertificate.java` for stronger entropy. (Added documentation in `ProxyModule.java` explaining why `java.util.Random` is intentionally retained there for metrics sampling).
3. Deserialization: Hardened `SerializeUtils.java` by injecting an `ObjectInputFilter` into the `ObjectInputStream`, restricting deserialization strictly to expected `google.registry` classes and standard Java collections.

proxy/src/main/java/google/registry/proxy/ProxyModule.java

@@ -383,6 +383,10 @@ public class ProxyModule {
   @Singleton
   @Provides
   static Random provideRandom() {
+    // Note: We intentionally use java.util.Random instead of SecureRandom here.
+    // This Random instance is injected into the hot path of the proxy exclusively for
+    // stochastic metrics sampling. Using SecureRandom would introduce severe lock
+    // contention and exhaust the system entropy pool under high load, causing DoS.
     return new Random();
   }