Support Desktop Actions

## Problem
For elementary 6 onward there isn't a mechanism to adding Contracts as we could in 5.1 to work, mostly because of how apps are sandboxed by flatpak.
This means for example my [Resizer app](https://github.com/peteruithoven/resizer) can't add a _Resize images_ option to the right click menu in Files. [More examples](https://github.com/elementary/contractor#examples).

## Proposal

Quoting @davidmhewitt:
> I believe the current plan is to work on expanding the XDG desktop file spec to allow [Desktop Actions](https://specifications.freedesktop.org/desktop-entry-spec/latest/ar01s11.html) to take file/URI arguments. Then we don't need the contractor service at all, and it will be standards compliant to work on other DEs/distros.

Is this something Files can already support?