feat: inject unified data migrations in dual writer (#114138)

* feat: draft changes for on-prem unified migration

* feat: further draft changes for on-prem unified migration

* fix: remove some tbis

* refactor: rename

* fix: another approach

* fix: background service related issues

* fix: address comments

* fix: make gen-go

* fix: background service related issues

* feat: refactor dual writer and legacy migrator

* fix: minor issues

* feat: working version in oss

* fix: wire

* fix: revert test data override

* fix: enterprise related issues

* chore: add todo

* fix: revert dual writer method

* fix: lint

* chore: logger format

* fix: reduce log level

* fix: log change

* fix: disable

* fix: address comments

* fix: return error on dual writer service

* fix: merge conflict

---------

Co-authored-by: Rafael Paulovic <rafael.paulovic@grafana.com>

pkg/storage/unified/migrations/contract/migrations.go

@@ -0,0 +1,12 @@
+package contract
+
+import (
+	"github.com/grafana/grafana/pkg/registry"
+)
+
+// UnifiedStorageMigrationService provides unified storage migrations as a background service.
+// This interface is defined in a separate package to avoid import cycles between
+// the migrations implementation and packages that need to depend on it (like dualwrite).
+type UnifiedStorageMigrationService interface {
+	registry.BackgroundService
+}