Add Share action to Dolphin context menu

Summary:
Use Purpose to enable sharing files using various services. Other KDE applications like Okular and Spectacle
already have that capability.

This feature overlaps with the various FileItemActionPlugins but is more generic as it is used by multiple
applications. At least some FileItemActionPlugins, such as the "Send via Bluetooth" plugin, should become
Purpose plugins instead. In other cases, such as KDE Connect, both approaches are supported at the moment.

Test Plan:
Select one or multiple files, right-click, Share, select e.g. Telegram or KDE Connect, check for
successful transfer.

Reviewers: apol, Dolphin, elvisangelaccio

Reviewed By: Dolphin, elvisangelaccio

Subscribers: markg, broulik, kde-frameworks-devel, elvisangelaccio, ngraham, apol, kfm-devel, Dolphin

Tags: Dolphin, Frameworks

Differential Revision: https://phabricator.kde.org/D13124