Add helper to parse an enum from query args & use it. (#14956)

The `parse_enum` helper pulls an enum value from the query string
(by delegating down to the parse_string helper with values generated
from the enum).

This is used to pull out "f" and "b" in most places and then we thread
the resulting Direction enum throughout more code.

tests/rest/admin/test_event_reports.py

@@ -280,7 +280,10 @@ class EventReportsTestCase(unittest.HomeserverTestCase):
 
         self.assertEqual(400, channel.code, msg=channel.json_body)
         self.assertEqual(Codes.INVALID_PARAM, channel.json_body["errcode"])
-        self.assertEqual("Unknown direction: bar", channel.json_body["error"])
+        self.assertEqual(
+            "Query parameter 'dir' must be one of ['b', 'f']",
+            channel.json_body["error"],
+        )
 
     def test_limit_is_negative(self) -> None:
         """