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.

ElementX/Sources/Screens/Settings/DeveloperOptionsScreen/View/DeveloperOptionsScreen.swift

@@ -120,7 +120,7 @@ struct DeveloperOptionsScreen: View {
                 Text("This setting controls how end-to-end encryption (E2EE) keys are exchanged. Enabling it will prevent the inclusion of devices that have not been explicitly verified by their owners.")
             }
             
-            Section("Element Call remote URL override") {
+            Section {
                 TextField("Leave empty to use EC locally", text: $elementCallURLOverrideString)
                     .autocorrectionDisabled(true)
                     .autocapitalization(.none)
@@ -133,6 +133,13 @@ struct DeveloperOptionsScreen: View {
                             context.elementCallBaseURLOverride = url
                         }
                     }
+                Toggle(isOn: $context.nativeCallEnabled) {
+                    Text("Experimental native calls")
+                }
+            } header: {
+                Text("Calls")
+            } footer: {
+                Text("The URL override only applies to calls run in the Element Call web view, which native calls replace.")
             }
             
             Section("Notifications") {