Cached avatar support (from Rust) (#633)

* loading a cached avatar at first, and then actually doing the request so that the avatar can still be displayed without connection

* improved the code

* fix + changelog

* policy on how to fetch the avatar url added

* code improvements

* addressed al PR comments

* improved the error handling

* improved the async task code

* fixed a typo

* fixed another typo

ElementX/Sources/Services/Client/MockClientProxy.swift

@@ -29,11 +29,15 @@ class MockClientProxy: ClientProxyProtocol {
     var visibleRoomsSummaryProvider: RoomSummaryProviderProtocol? = MockRoomSummaryProvider()
     
     var allRoomsSummaryProvider: RoomSummaryProviderProtocol? = MockRoomSummaryProvider()
+
+    var avatarURLPublisher: AnyPublisher<URL?, Never> { Empty().eraseToAnyPublisher() }
     
     internal init(userID: String, roomSummaryProvider: RoomSummaryProviderProtocol? = MockRoomSummaryProvider()) {
         self.userID = userID
         visibleRoomsSummaryProvider = roomSummaryProvider
     }
+
+    func loadUserAvatarURL() async { }
     
     func startSync() { }
     
@@ -56,10 +60,6 @@ class MockClientProxy: ClientProxyProtocol {
         .success("User display name")
     }
     
-    func loadUserAvatarURL() async -> Result<URL, ClientProxyError> {
-        .failure(.failedRetrievingAvatarURL)
-    }
-    
     func accountDataEvent<Content>(type: String) async -> Result<Content?, ClientProxyError> where Content: Decodable {
         .failure(.failedRetrievingAccountData)
     }