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

Riot/Modules/Common/SwiftUI/Inject.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
+
+@propertyWrapper struct Inject<Value> {
+    
+    static subscript<T: Injectable>(
+        _enclosingInstance instance: T,
+        wrapped wrappedKeyPath: ReferenceWritableKeyPath<T, Value>,
+        storage storageKeyPath: ReferenceWritableKeyPath<T, Self>
+    ) -> Value {
+        get {
+            let v: Value = instance.dependencies.resolve()
+            return v
+        }
+        set {
+            fatalError()
+        }
+    }
+    
+    @available(*, unavailable,
+    message: "@Published can only be applied to classes"
+    )
+    var wrappedValue: Value {
+        get { fatalError() }
+        set { fatalError(" \(newValue)" ) }
+    }
+}