Add 'on_user_search' callback to third_party_rules callback

[awesome-michael]

Add 'on_user_search' callback to the third_party_rules section of the module API.
This allows to write modules that can trigger on search requests on the user directory and alter the results.
Possible use cases:

• Filter or group user search results
• Exclude MxIDs from the user search results
• Include MxIDs in the user search results even if they do not match the search criteria (e.g. a helper Bot)

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[clokep]

Would probably make more sense to do this at the handler level (

synapse/synapse/handlers/user_directory.py

Lines 143 to 175 in c360da0

	async def search_users(
	self, user_id: str, search_term: str, limit: int
	) -> SearchResult:
	"""Searches for users in directory
	Returns:
	dict of the form::
	{
	"limited": <bool>, # whether there were more results or not
	"results": [ # Ordered by best match first
	{
	"user_id": <user_id>,
	"display_name": <display_name>,
	"avatar_url": <avatar_url>
	}
	]
	}
	"""
	results = await self.store.search_user_dir(
	user_id, search_term, limit, self.show_locked_users
	)
	# Remove any spammy users from the results.
	non_spammy_users = []
	for user in results["results"]:
	if not await self._spam_checker_module_callbacks.check_username_for_spam(
	user, user_id
	):
	non_spammy_users.append(user)
	results["results"] = non_spammy_users
	return results

) so that it also applies to the admin API?