Add experimental native calls behind a developer flag (#6159)

Add the element-call-ios dependency and a developer option for native calls

The package carries the call UI and the matrix-rust-rtc media stack, pinned by
exact version the way the SDK is, with the local path kept commented out for
working against a checkout. Its media xcframework has no Intel simulator slice
and a package cannot say so for the projects that consume it, hence the
excluded architecture at the project level.

The developer option is off by default and nothing reads it yet. It is placed
next to the Element Call URL override, which only applies to the web view that
native calls replace.

UnitTests/Sources/TestUtilities/ManualClock.swift

@@ -45,6 +45,12 @@ final nonisolated class ManualClock: Clock {
         state.withLock { $0.now }
     }
     
+    /// Whether a sleep is waiting on ``advance(by:)``, which is how a test asserts that a timer has
+    /// been cancelled rather than advancing the clock and watching for what it doesn't do.
+    var hasPendingSleep: Bool {
+        state.withLock { $0.sleep != nil }
+    }
+    
     /// Suspends until ``advance(by:)`` moves the clock to `deadline`, or the calling task is
     /// cancelled, in which case this throws a `CancellationError`. Deadlines that have already
     /// passed return without suspending. `tolerance` is ignored.