feat(subsonic): support timeOffset in stream.view (#384)

as per
https://github.com/opensubsonic/open-subsonic-api/pull/54
https://github.com/opensubsonic/open-subsonic-api/discussions/21

dont cache partial transcodes

add a transcode seek test

server/ctrlsubsonic/handlers_common.go

@@ -38,7 +38,9 @@ func (c *Controller) ServePing(_ *http.Request) *spec.Response {
 
 func (c *Controller) ServeGetOpenSubsonicExtensions(_ *http.Request) *spec.Response {
 	sub := spec.NewResponse()
-	sub.OpenSubsonicExtensions = &spec.OpenSubsonicExtensions{}
+	sub.OpenSubsonicExtensions = &spec.OpenSubsonicExtensions{
+		{Name: "transcodeOffset", Versions: []int{1}},
+	}
 	return sub
 }