[subsonic] coverArt missing from getArtists response

What is the problem you are facing?

The getArtists endpoint in the subsonic API does not return the coverArt attribute, making it impossible for a client application to show a nice list of artists with images.

What are the possible drawbacks or issues with the requested changes?

Crummy looking artist list in client apps.

Context

Here is a snippet from the getArtists response from the latest version of Funkwhale:

            {
              "id": 2,
              "name": "Becays",
              "albumCount": 1
            },

Compared to what a Subsonic server returns:

	{
          albumCount = 2;
          coverArt = "ar-94";
          id = 94;
          name = "a-ha";
        },

Not the missing coverArt in the Funkwhale response.

The coverArt attribute is marked optional in the spec for the subsonic api: http://www.subsonic.org/pages/inc/api/schema/subsonic-rest-api-1.16.1.xsd

But it would a great addition to Funkwhales Subsonic support ;-)