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/RoomScreen/ComposerToolbar/ComposerToolbarViewModel.swift

@@ -216,8 +216,8 @@ final class ComposerToolbarViewModel: ComposerToolbarViewModelType, ComposerTool
         case .attach(let attachment):
             state.bindings.composerFocused = false
             actionsSubject.send(.attach(attachment))
-        case .handlePasteOrDrop(let provider):
-            actionsSubject.send(.handlePasteOrDrop(provider: provider))
+        case .handlePasteOrDrop(let providers):
+            actionsSubject.send(.handlePasteOrDrop(providers: providers))
         case .enableTextFormatting:
             state.bindings.composerFormattingEnabled = true
             state.bindings.composerFocused = true