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.

UnitTests/Sources/MediaUploadPreviewScreenViewModelTests.swift

@@ -173,7 +173,7 @@ class MediaUploadPreviewScreenViewModelTests: XCTestCase {
             clientProxy.underlyingMaxMediaUploadSize = maxUploadSizeResult
         }
         
-        viewModel = MediaUploadPreviewScreenViewModel(url: url,
+        viewModel = MediaUploadPreviewScreenViewModel(mediaURLs: [url],
                                                       title: "Some File",
                                                       isRoomEncrypted: true,
                                                       shouldShowCaptionWarning: true,