Fixes#1592.
## Summary
Enables the Notification Content extension now that it works in Xcode 13.
## Any other notes
This is an entirely tooling enabling of this feature, as the extension works on macOS 11. There is a gotcha: the Notification Service Extension attachments are inaccessible. Additionally, macOS always displays the attached-by-us images/movies, which is a frustrating difference in behavior between macOS and iOS, but makes this particular deficiency less noticeable. Filed as FB9638431.
## Summary
Allows attachments to be loaded on-the-fly in the content extension if they are not present.
## Link to pull request in Documentation repository
Documentation: home-assistant/companion.home-assistant#510
## Any other notes
- Adds `lazy` as an option to the `attachment` dictionary. When true, this prevents the attachment from being downloaded except when displayed in the content extension.
This is similar, but not identical, to hide-thumbnail. Hiding the thumbnail still uses the internal storage of the notifications which are limited to e.g. 50mb. However, we'd still want to pre-load those attachments even if the thumbnail is hidden.
- Retries non-lazy attachments which failed due to a network error when the content extension opens.
Fixes#1591.
## Summary
Adds support for using video and audio notification attachments in the content extension, including audio playback for both.
## Screenshots
https://user-images.githubusercontent.com/74188/115503572-7c70e900-a22b-11eb-970b-b4e9c2225986.mov
## Any other notes
Dynamic actions require the 'DYNAMIC' category but our content extension causes attachments to not appear and we want every possible feature to work with just this category.
