Add recursion_depth to /relations if recursing (#16775)

This is an extra response parameter just added to MSC3981. In the
current impl, the recursion depth is always 3, so this just returns a
static 3 if the recurse parameter is supplied.

changelog.d/16775.bugfix

@@ -0,0 +1 @@
+Adds the recursion_depth parameter to the response of the /relations endpoint if MSC3981 recursion is being performed.

synapse/handlers/relations.py

@@ -180,6 +180,10 @@ class RelationsHandler:
                 config=serialize_options,
             ),
         }
+
+        if recurse:
+            return_value["recursion_depth"] = 3
+
         if include_original_event:
             # Do not bundle aggregations when retrieving the original event because
             # we want the content before relations are applied to it.