Add refreshed_at and created_at fields to gateway_metadata

src/service/mobile_config.proto

@@ -25,6 +25,12 @@ message gateway_metadata {
   // The res12 h3 index asserted address of the gateway as a string
   // where an unasserted gateway returns an empty string
   string location = 2;
+  // The unix epoch timestamp (in seconds)
+  // when the data was read from chain and written to the db
+  uint64 refreshed_at = 3
+  // The unix epoch timestamp (in seconds)
+  // when the gateway was first added to the database.
+  uint64 created_at = 4;
 }
 
 message gateway_info {