Multi file uploads (#4358)

* Allow MediaPickerScreen users to select the media selection mode (single or multiple)

* Fix cancellation

* Add support for multiple media URLs on the MediaUploadPreviewScreen.

* Support processing more URLs on the `MediaUploadingPreprocessor` and sending more on the `MediaUploadPreviewScreen`

* Add feature flag for `multipleAttachmentUploadEnabled`

* Add a label showing the current preview item index in the MediaUploadPreviewScreen

* Add support for dragging and dropping or pasting multiple items at the same time.

* Support sharing more than one file through the share extension.

* Limit the number of items that can be shared in one go to 5.

* Fix unit tests

* Fix incorrect fatal error when dealing with single selection media pickers.

* Document the `multipleAttachmentUploadEnabled` usage in the context of the MediaPicker.

* Use a task group for processing selected media in the photo library picker.

* Use a task group for processing multiple selected media in the MediaUploadingPreprocessor

* Switch the maximum number of items that can be shared to 10.

* Allow multiple items to be pasted at the same time.

ElementX/Sources/Screens/Timeline/TimelineModels.swift

@@ -18,7 +18,7 @@ enum TimelineViewModelAction {
     case displayDocumentPicker
     case displayLocationPicker
     case displayPollForm(mode: PollFormMode)
-    case displayMediaUploadPreviewScreen(url: URL)
+    case displayMediaUploadPreviewScreen(mediaURLs: [URL])
     case displaySenderDetails(userID: String)
     case displayMessageForwarding(forwardingItem: MessageForwardingItem)
     case displayMediaPreview(TimelineMediaPreviewViewModel)
@@ -63,7 +63,7 @@ enum TimelineViewAction {
     case displayReadReceipts(itemID: TimelineItemIdentifier)
     case displayThread(itemID: TimelineItemIdentifier)
     
-    case handlePasteOrDrop(provider: NSItemProvider)
+    case handlePasteOrDrop(providers: [NSItemProvider])
     case handlePollAction(TimelineViewPollAction)
     case handleAudioPlayerAction(TimelineAudioPlayerAction)