feat(snap): ability to specify slot properties

[feugy]

What's in there?

• The ability to specify snap's slot properties (fixes [Snap]: can not specify slot details #5339)

This is required to allow snap-packaged app declaring mpris slots.

How to test?

Compile source and schema, then TEST_FILES=snapTest yarn test.

Note to reviewers

I'm currently developing a portable music player with electron named melodie.
One very basic feature is the usage of MediaMetadata, which makes the operating system aware of which track is currently playing, and gives it a change to pause, and jump to next or previous.

On linux systems, this integrations is use Media Player Remote Interface Specification (mpris): the application sends DBus commands to interact with the OS.

This works perfectly fine with all the packager I've tested, except with snap.

When running the snap in devmode or with classic confinement, Linux can interact with the player controls.
But when using strict confinement, Linux is not aware of it.
The snappy-debug tool spot the issue pretty quickly:

= AppArmor =
Time: Sep 26 20:57:04
Log: apparmor="DENIED" operation="dbus_bind"  bus="session" name="org.mpris.MediaPlayer2.chromium.instance38417" mask="bind" pid=38417 label="snap.melodie.melodie"
DBus access
Suggestion:
* use 'mpris' slot (https://github.com/snapcore/snapd/wiki/Interfaces#mpris)

After two days of try-and-fail research and web crawling it appears that:

• the mpris slot needs a name attribute. When it's not provided, the name is the application name (melodie in my case)
• electron use chromium, but chromium does not give control over the named use. It is hardcoded as chromium

I initially though I could hardcode the mpris name as long as someone specifies mpris in electron-builder's configuration, but then I changed to something generic: there are other slots that require parameters.

[feugy]

Hello there.

Is there anything I could provide to get this PR reviewed?
I'd like to release my application, and I can not until I get this fixed.

Thanks a million.

[feugy]

Thanks a million Develar!