Storage: Customise setting dual writing modes (#87668)

* Add feature toggles for mode 2 and 3 playlist dual writing
* Make current mode customised based on kind
* Check feature flags when initialising dual writer
* Fix linting
* Refactor NewDualWriter

.golangci.toml

@@ -76,6 +76,7 @@ allow = [
   "github.com/grafana/grafana/pkg/apimachinery",
   "github.com/grafana/grafana/pkg/apiserver",
   "github.com/grafana/grafana/pkg/services/apiserver/utils",
+  "github.com/grafana/grafana/pkg/services/featuremgmt",
 ]
 deny = [
   { pkg = "github.com/grafana/grafana/pkg", desc = "apiserver is not allowed to import grafana core" }