Support for PSTN users and call rooms

[simonwiedmer]

Feature is described here: element-hq/element-ios#5782

This part of the feature needs support and implementation in the SDK:

As the PSTN bridge creates a matrix user as a proxy for each PSTN number, it would be needed to distinguish this kind of special matrix user from a normal matrix user. Based on the presence of such a special matrix user within a room, the room itself can then be considered a "call" room to be presented in the call list and be excluded from being presented in the list of direct rooms or group rooms.