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/Services/Client/ClientProxy.swift

@@ -8,6 +8,7 @@
 
 @preconcurrency import Combine
 import CryptoKit
+import ElementCall
 import Foundation
 import MatrixRustSDK
 import OrderedCollections
@@ -374,6 +375,10 @@ class ClientProxy: ClientProxyProtocol {
         }
     }
     
+    func makeNativeCallTransport() -> ElementCallMatrixTransportProtocol? {
+        (client as? Client).flatMap { ElementCallSDKTransport(client: $0) }
+    }
+    
     var isLoginWithQRCodeSupported: Bool {
         get async {
             do {

ElementX/Sources/Services/Client/ClientProxyProtocol.swift

@@ -7,6 +7,7 @@
 //
 
 import Combine
+import ElementCall
 import Foundation
 import MatrixRustSDK
 
@@ -171,6 +172,9 @@ protocol ClientProxyProtocol: AnyObject {
     var isReportRoomSupported: Bool { get async }
     var isLiveKitRTCSupported: Bool { get async }
     
+    /// Builds a Matrix transport for the call package, `nil` when not backed by a real SDK client.
+    func makeNativeCallTransport() -> ElementCallMatrixTransportProtocol?
+    
     var isLoginWithQRCodeSupported: Bool { get async }
     
     var maxMediaUploadSize: Result<UInt, ClientProxyError> { get async }