Break SessionSource out of SessionMetadata and rename it EppRequestSource.

The "SessionSource" has nothing to do with sessions (and it's often
used in sessionless contexts). What it does indicate is the endpoint
used to make the request.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=125295224

java/google/registry/flows/EppRequestHandler.java

@@ -42,13 +42,19 @@ public class EppRequestHandler {
   public void executeEpp(
       SessionMetadata sessionMetadata,
       TransportCredentials credentials,
+      EppRequestSource eppRequestSource,
       boolean isDryRun,
       boolean isSuperuser,
       byte[] inputXmlBytes) {
     try {
       response.setPayload(new String(
           eppController.handleEppCommand(
-              sessionMetadata, credentials, isDryRun, isSuperuser, inputXmlBytes), UTF_8));
+              sessionMetadata,
+              credentials,
+              eppRequestSource,
+              isDryRun,
+              isSuperuser,
+              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