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

Riot/Modules/Common/SwiftUI/DependencyContainer.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
+
+struct DependencyContainer {
+
+    private var dependencyList: [String: Any] = [:]
+    
+    func resolve<T>() -> T {
+        let key = String(describing: T.self)
+        guard let t = dependencyList[key] as? T else {
+            fatalError("No provider registered for type \(T.self)")
+        }
+        return t
+    }
+    
+    mutating func register<T>(dependency: T) {
+        let key = String(describing: T.self)
+        dependencyList[key] = dependency
+    }
+}