Map `notifications.room` from the `m.room.power_levels` state event

In the context of https://github.com/element-hq/element-x-android/issues/4761, applications using the Rust SDK must be able to know the value of `notifications.room` from the `m.room.power_levels` state event.

This is currently not mapped in `RoomPowerLevels` object that the application can access.

See https://spec.matrix.org/latest/client-server-api/#mroompower_levels

Start by locating the Rust SDK's RoomPowerLevels object and compare its mapped fields with the Matrix m.room.power_levels specification linked in the issue. Add the notifications.room value to the exposed mapping and verify that the object now provides the specified value.

Written by the indexing model from the issue text.