Implement MSC3952: Intentional mentions (#14823)

MSC3952 defines push rules which searches for mentions in a list of
Matrix IDs in the event body, instead of searching the entire event
body for display name / local part.

This is implemented behind an experimental configuration flag and
does not yet implement the backwards compatibility pieces of the MSC.

stubs/synapse/synapse_rust/push.pyi

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Any, Collection, Dict, Mapping, Optional, Sequence, Tuple, Union
+from typing import Any, Collection, Dict, Mapping, Optional, Sequence, Set, Tuple, Union
 
 from synapse.types import JsonDict
 
@@ -46,6 +46,7 @@ class FilteredPushRules:
         msc1767_enabled: bool,
         msc3381_polls_enabled: bool,
         msc3664_enabled: bool,
+        msc3952_intentional_mentions: bool,
     ): ...
     def rules(self) -> Collection[Tuple[PushRule, bool]]: ...
 
@@ -55,6 +56,8 @@ class PushRuleEvaluator:
     def __init__(
         self,
         flattened_keys: Mapping[str, str],
+        user_mentions: Set[str],
+        room_mention: bool,
         room_member_count: int,
         sender_power_level: Optional[int],
         notification_power_levels: Mapping[str, int],