feat(snap): add config interface with unique identifier

[MonicaisHer]

The device service snaps all have device-config as the config interface identifier. This is fine as long as we have one config provider per device service snaps. The identical names however prevent the user from having a single provider for multiple snaps.

Setting up a content provider to seed multiple device snaps would result in the following error:

connection not allowed by slot rule of interface "content"

It turns out that for manual connections (via snap connect or gadget's connections), the slot and plug name must be the same. From the docs:

A slot and a plug can only be connected if they have the same interface name.
https://snapcraft.io/docs/content-interface

This PR deprecates the old config interface in favor of one with a unique identifier.

Signed-off-by: Mengyi Wang mengyi.wang@canonical.com

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