Decouple SessionMetadata and TransportCredentials

TransportCredentials are per-request, not per-session, and
there's no reason to carry them within SessionMetadata.

While I'm in here, get rid of "null" credentials.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=125202213

java/google/registry/flows/EppRequestHandler.java

@@ -39,10 +39,13 @@ public class EppRequestHandler {
   @Inject EppRequestHandler() {}
 
   /** Handle an EPP request and write out a servlet response. */
-  public void executeEpp(SessionMetadata sessionMetadata, byte[] inputXmlBytes) {
+  public void executeEpp(
+      SessionMetadata sessionMetadata,
+      TransportCredentials credentials,
+      byte[] inputXmlBytes) {
     try {
       response.setPayload(new String(
-          eppController.handleEppCommand(sessionMetadata, inputXmlBytes), UTF_8));
+          eppController.handleEppCommand(sessionMetadata, credentials, inputXmlBytes), UTF_8));
       response.setContentType(APPLICATION_EPP_XML);
       // Note that we always return 200 (OK) even if the EppController returns an error response.
       // This is because returning an non-OK HTTP status code will cause the proxy server to