`snap` should provide functionality to find packages with needed plug or connector

[Norbert]

Steps to reproduce:
1. Install any supported Ubuntu version
2. Try to use `snap` command to find packages with needed plug or connector

Expected results:
* snap has functionality to find packages with needed plug or connector

Actual results:
* snap do not have functionality to find packages with needed plug or connector

Note:
the problem is actual because of the fact that user will not know about supported plugs or connector until he/she install the snap (see https://askubuntu.com/q/1046585/66509 and https://askubuntu.com/q/1034030/66509 ) . For example it is hard to find Snap package with `:removable-media` plug or connector. Snapcart.io does not have this functionality (see https://github.com/canonical-websites/snapcraft.io/issues/642 ) .

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: snapd 2.32.9
ProcVersionSignature: Ubuntu 4.4.0-127.153-generic 4.4.128
Uname: Linux 4.4.0-127-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Jun 14 19:26:49 2018
InstallationDate: Installed on 2018-01-04 (160 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
SourcePackage: snapd
UpgradeStatus: No upgrade log present (probably fresh install)

[John Lenton (chipaca)]

Yes, we should.

You can find this information out, but not in a particularly friendly way.
See https://forum.snapcraft.io/t/show-interfaces-of-snap-before-installation/13252/3?u=chipaca for more info.