MT#59363 /api/subscriberregistrations change socket field to Text

* socket field is changed to text for the API endpoint as by default
  in the LocationEntry form that is used by the UI the field is
  a prebuilt options list that comes from config.yml extra_sockets,
  and that severely limits the usage for the API.

Change-Id: I71ee1f77dbf458c12655f840282375cee62b7534

lib/NGCP/Panel/Form/Subscriber/LocationEntryAPI.pm

@@ -41,6 +41,15 @@ has_field 'received' => (
     },
 );
 
+has_field 'socket' => (
+    type => 'Text',
+    required => 0,
+    element_attr => {
+        rel => ['tooltip'],
+        title => ['Points to the LB interface from which the incoming calls to this registration should be sent out.']
+    },
+);
+
 1;
 
 __END__