[ios] [bookmarks] Remove observer pattern during the category files exporting

[kirylkaveryn]

For now to handle thefile sharing OM uses the observing pattern tightened to the MWMBookmarksManagers, that is too complex for this straightforward task:

1. subscribe for the observarion
2. register observer
3. send sharing message
4. wait for sharing result
5. handle sharing result
6. notify observers
7. unsubscribe from observing

This is too complex because the c++ method that we interact with uses the completion handlers to return sharing results and there is no sense in wrapping the completion handle into observation:

void BookmarkManager::PrepareFileForSharing(kml::GroupIdCollection && categoriesIds, SharingHandler && handler)

This PR converts c++ completion handler into the objc and returns to the caller so we can share files easily with the MWMBookmarksManagers and handle result without additional overhead:

- (void)shareCategory:(MWMMarkGroupID)groupId completion:(SharingResultCompletionHandler)completion;
- (void)shareAllCategoriesWithCompletion:(SharingResultCompletionHandler)completion;

https://github.com/organicmaps/organicmaps/assets/79797627/7ceabaa2-0880-41e4-bfed-28c565911950

[kirylkaveryn]

Thanks, the functionality did not change, right? All cases (including errors) are properly handled now in the Bookmarks and Tracks dialog, right?

Yes! Tested on iPhone 11Pro (17.2) and iPhone 6 (12.5).

[kirylkaveryn]

Updated with documentation!