Add Unix socket support for Redis connections (#15644)

Adds a new configuration setting to connect to Redis via a Unix
socket instead of over TCP. Disabled by default.

stubs/txredisapi.pyi

@@ -61,6 +61,9 @@ def lazyConnection(
 # most methods to it via ConnectionHandler.__getattr__.
 class ConnectionHandler(RedisProtocol):
     def disconnect(self) -> "Deferred[None]": ...
+    def __repr__(self) -> str: ...
+
+class UnixConnectionHandler(ConnectionHandler): ...
 
 class RedisFactory(protocol.ReconnectingClientFactory):
     continueTrying: bool