Implement support for ElementCall through Rust side widgets (#1906)

* Implement support for ElementCall through Rust side widgets

* Add Element Call feature flag

* Fix the unit tests

* Address PR comments

ElementX/Sources/Services/ElementCall/ElementCallWidgetDriverProtocol.swift

@@ -0,0 +1,42 @@
+//
+// Copyright 2023 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+import Combine
+import Foundation
+
+enum ElementCallWidgetDriverError: Error {
+    case roomInvalid
+    case failedBuildingCallURL
+    case failedBuildingWidgetSettings
+    case failedBuildingWidgetDriver
+    case failedParsingCallURL
+    case driverNotSetup
+}
+
+enum ElementCallWidgetDriverAction {
+    case callEnded
+    case errorReceived(ElementCallWidgetDriverError)
+}
+
+// sourcery: AutoMockable
+protocol ElementCallWidgetDriverProtocol {
+    var messagePublisher: PassthroughSubject<String, Never> { get }
+    var actions: AnyPublisher<ElementCallWidgetDriverAction, Never> { get }
+    
+    func start(baseURL: URL, clientID: String) async -> Result<URL, ElementCallWidgetDriverError>
+    
+    func sendMessage(_ message: String) async -> Result<Bool, ElementCallWidgetDriverError>
+}