Refactor Agents to be singleton

Initial work for Last.fm scrobbler

core/scrobbler/interfaces.go

@@ -0,0 +1,20 @@
+package scrobbler
+
+import (
+	"context"
+	"time"
+
+	"github.com/navidrome/navidrome/model"
+)
+
+type Scrobble struct {
+	Track     *model.MediaFile
+	TimeStamp *time.Time
+}
+
+type Scrobbler interface {
+	NowPlaying(context.Context, *model.MediaFile) error
+	Scrobble(context.Context, []Scrobble) error
+}
+
+type Constructor func(ds model.DataStore) Scrobbler