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.

synapse/handlers/account_data.py

@@ -314,7 +314,7 @@ class AccountDataEventSource(EventSource[int, JsonDict]):
     def __init__(self, hs: "HomeServer"):
         self.store = hs.get_datastores().main
 
-    def get_current_key(self, direction: str = "f") -> int:
+    def get_current_key(self) -> int:
         return self.store.get_max_account_data_stream_id()
 
     async def get_new_events(