feat: Add Share option for artists, albums, and tracks

This feature is only available on the Subsonic media provider. Using a
Jellyfin server will make Supersonic hide the "Share" option.

* Artists: Option available in grid view and in Artist view.
* Albums: Option available in grid view and in Album view.
* Tracks: Option available in tracklist, when a single track is selected.

backend/mediaprovider/mediaprovider.go

@@ -3,6 +3,7 @@ package mediaprovider
 import (
 	"image"
 	"io"
+	"net/url"
 	"strings"
 )
 
@@ -148,6 +149,10 @@ type SupportsRating interface {
 	SetRating(params RatingFavoriteParameters, rating int) error
 }
 
+type SupportsSharing interface {
+	CreateShareURL(id string) (*url.URL, error)
+}
+
 type JukeboxProvider interface {
 	JukeboxStart() error
 	JukeboxStop() error