feat: knocking

You may notice that we do no database migration for populating the state cache for knocking.
This is because that in all the places where we use the state cache, it doesn't make a difference:
- For local users, the clients wouldn't have been able to knock on rooms, as the `/knock`
  endpoint wasn't implemented yet, and I am not aware of any client which tries to knock over
  `/state`, as it would fail if the server is not currently in the room
- It is not used for remote users

src/database/mod.rs

@@ -99,6 +99,8 @@ pub struct KeyValueDatabase {
     pub(super) roomuseroncejoinedids: Arc<dyn KvTree>,
     pub(super) userroomid_invitestate: Arc<dyn KvTree>, // InviteState = Vec<Raw<AnyStrippedStateEvent>>
     pub(super) roomuserid_invitecount: Arc<dyn KvTree>, // InviteCount = Count
+    pub(super) userroomid_knockstate: Arc<dyn KvTree>, // KnockState = Vec<Raw<AnyStrippedStateEvent>>
+    pub(super) roomuserid_knockcount: Arc<dyn KvTree>, // KnockCount = Count
     pub(super) userroomid_leftstate: Arc<dyn KvTree>,
     pub(super) roomuserid_leftcount: Arc<dyn KvTree>,
 
@@ -313,6 +315,8 @@ impl KeyValueDatabase {
             roomuseroncejoinedids: builder.open_tree("roomuseroncejoinedids")?,
             userroomid_invitestate: builder.open_tree("userroomid_invitestate")?,
             roomuserid_invitecount: builder.open_tree("roomuserid_invitecount")?,
+            userroomid_knockstate: builder.open_tree("userroomid_knockstate")?,
+            roomuserid_knockcount: builder.open_tree("roomuserid_knockcount")?,
             userroomid_leftstate: builder.open_tree("userroomid_leftstate")?,
             roomuserid_leftcount: builder.open_tree("roomuserid_leftcount")?,