Do avatar placeholder in SwiftUI, Add AvatarViewModel, Add dependency injection for MxMediaManager dependency.

Riot/Modules/Common/SwiftUI/DependencyContainerKey.swift

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+//
+// Copyright 2021 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 Foundation
+import SwiftUI
+
+private struct DependencyContainerKey: EnvironmentKey {
+    static let defaultValue = DependencyContainer()
+}
+
+@available(iOS 14.0, *)
+extension EnvironmentValues {
+    var dependencies: DependencyContainer {
+        get { self[DependencyContainerKey.self] }
+        set { self[DependencyContainerKey.self] = newValue }
+    }
+}
+
+/**
+ */
+@available(iOS 14.0, *)
+extension View {
+    func setDependencies(_ container: DependencyContainer) -> some View {
+        environment(\.dependencies, container)
+    }
+    
+    func addDependency<T>(_ dependency: T) -> some View {
+        transformEnvironment(\.dependencies) { container in
+            container.register(dependency: dependency)
+        }
+    }
+}