Allow snaps to query interface connection status directly from snapd

[林博仁(Buo-ren Lin)]

Detecting whether certain security confinement interface is connected to the snap is not a trivial task:

* Sometimes certain access is granted by multiple interfaces, whether it's available or not didn't indicate whether a certain interface is connected
* One must have a certain level of Apparmor profile knowledge and check out the source code at https://github.com/snapcore/snapd/blob/master/interfaces/builtin to determine the test criterion, not all potential packagers has this kind of experience
* The testing of the connection will trigger security denials in the system log, which may be considered "abnormal" or even "malicious" by the users

As the connection info is already in snapd I would suggest exposing them directly via the `snapctl` command so one can simply write:

```bash
if ! snapctl is-connected _interface_name_; then
    echo "_interface_name_ is not connected!" >&2
fi
```

without caring much the implementation details.

[林博仁(Buo-ren Lin) (brlin)]

Discussion held at https://forum.snapcraft.io/t/bug-1809708-allow-snaps-to-query-interface-connection-status-directly-from-snapd/9147

[Paweł Stołowski (stolowski)]

I agree some way of interrogating snapctl will be useful. Thank you for the report.

One way around this limation and the problem of probing for effective permissions that you can already use is interface hooks - see https://snapcraft.io/docs/interface-hooks - in with particular with "connect-*" hooks you can keep track of what's connected to your snap.

[Ian Johnson (anonymouse67)]

In Paris we agreed that this would be `snapctl is-connected <plug-name>` (or slot-name)

[Paweł Stołowski (stolowski)]

Implemented with https://github.com/snapcore/snapd/pull/7771, should become available with snapd 2.43.