Pass room_config argument to user_may_create_room spam checker module callback (#18486)

This PR adds an additional `room_config` argument to the
`user_may_create_room` spam checker module API callback.

It will continue to work with implementations of `user_may_create_room`
that do not expect the additional parameter.

A side affect is that on a room upgrade the spam checker callback is
called *after* doing some work to calculate the state rather than
before. However, I hope that this is acceptable given the relative
infrequency of room upgrades.

docs/modules/spam_checker_callbacks.md

@@ -159,12 +159,19 @@ _First introduced in Synapse v1.37.0_
 
 _Changed in Synapse v1.62.0: `synapse.module_api.NOT_SPAM` and `synapse.module_api.errors.Codes` can be returned by this callback. Returning a boolean is now deprecated._ 
 
+_Changed in Synapse v1.132.0: Added the `room_config` argument. Callbacks that only expect a single `user_id` argument are still supported._
+
 ```python
-async def user_may_create_room(user_id: str) -> Union["synapse.module_api.NOT_SPAM", "synapse.module_api.errors.Codes", bool]
+async def user_may_create_room(user_id: str, room_config: synapse.module_api.JsonDict) -> Union["synapse.module_api.NOT_SPAM", "synapse.module_api.errors.Codes", bool]
 ```
 
 Called when processing a room creation request.
 
+The arguments passed to this callback are:
+
+* `user_id`: The Matrix user ID of the user (e.g. `@alice:example.com`).
+* `room_config`: The contents of the body of a [/createRoom request](https://spec.matrix.org/latest/client-server-api/#post_matrixclientv3createroom) as a dictionary.
+
 The callback must return one of:
   - `synapse.module_api.NOT_SPAM`, to allow the operation. Other callbacks may still 
     decide to reject it.

docs/spam_checker.md

@@ -63,7 +63,7 @@ class ExampleSpamChecker:
     async def user_may_invite(self, inviter_userid, invitee_userid, room_id):
         return True  # allow all invites
 
-    async def user_may_create_room(self, userid):
+    async def user_may_create_room(self, userid, room_config):
         return True  # allow all room creations
 
     async def user_may_create_room_alias(self, userid, room_alias):