Use our own JoinRule type which doesn't include the reserved .private case. (#5056)

* Stop using Rust's JoinRule.private in mocks.

It isn't used anywhere else (see MSC4413 for more context).

* Use our own JoinRule (and AllowRule) types.

* Update ElementX/Sources/Services/Room/JoinRule.swift

Co-authored-by: Stefan Ceriu <stefan.ceriu@gmail.com>

---------

Co-authored-by: Stefan Ceriu <stefan.ceriu@gmail.com>

ElementX/Sources/Services/Room/RoomInfoProxy.swift

@@ -135,7 +135,7 @@ struct RoomInfoProxy: RoomInfoProxyProtocol {
     }
 
     var joinRule: JoinRule? {
-        roomInfo.joinRule
+        roomInfo.joinRule.map(JoinRule.init)
     }
 
     var historyVisibility: RoomHistoryVisibility {
@@ -193,7 +193,7 @@ struct RoomPreviewInfoProxy: BaseRoomInfoProxyProtocol {
     }
     
     var joinRule: JoinRule? {
-        roomPreviewInfo.joinRule
+        roomPreviewInfo.joinRule.map(JoinRule.init)
     }
 
     var membership: Membership? {