[schema] Add a Client model keeping track of the sending client.

(imported from commit 31a430b1de14ce973addafd5d13ace049a8f8091)

api/zephyr_mirror.py

@@ -99,6 +99,7 @@ def send_humbug(zeph):
         elif isinstance(zeph[key], str):
             zeph[key] = zeph[key].decode("utf-8")
 
+    zeph['client'] = "zephyr_mirror"
     return humbug_client.send_message(zeph)
 
 def fetch_fullname(username):