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/Flow.java

@@ -43,6 +43,7 @@ public abstract class Flow {
   protected EppInput eppInput;
   protected SessionMetadata sessionMetadata;
   protected TransportCredentials credentials;
+  protected EppRequestSource eppRequestSource;
   protected Trid trid;
   protected DateTime now;
   protected byte[] inputXmlBytes;
@@ -103,6 +104,7 @@ public abstract class Flow {
       Trid trid,
       SessionMetadata sessionMetadata,
       TransportCredentials credentials,
+      EppRequestSource eppRequestSource,
       boolean isSuperuser,
       DateTime now,
       byte[] inputXmlBytes) throws EppException {
@@ -110,6 +112,7 @@ public abstract class Flow {
     this.trid = trid;
     this.sessionMetadata = sessionMetadata;
     this.credentials = credentials;
+    this.eppRequestSource = eppRequestSource;
     this.now = now;
     this.isSuperuser = isSuperuser;
     this.inputXmlBytes = inputXmlBytes;