Started the big refactor to extract common logic from engine package (Subsonic only) to core package (more generic)

core/common.go

@@ -0,0 +1,15 @@
+package core
+
+import (
+	"context"
+	"github.com/deluan/navidrome/model/request"
+)
+
+func userName(ctx context.Context) string {
+	if user, ok := request.UserFrom(ctx); !ok {
+		return "UNKNOWN"
+	} else {
+		return user.UserName
+	}
+}
+