Unified Storage: Adds pruner to kv backend (#110549)

* WIP adding pruner to kv store impl

* pruner only keeps 20 most recent versions

* ignore grafana-kv-data folder

* extracts some stuff to pruner.go file. Adds tests. Adds kvBackendOptions.

* update logging, comments, exports kvbackendoptions fields

* update nooppruner ref

* fixes field casing in test

* fix test

* linter fixes

* remove comment

* make KvStorageBackend private

* Adds pruner key validation and tests. Fixes broken tests.

* update error message when validating pruner key

.gitignore

@@ -245,3 +245,7 @@ public/mockServiceWorker.js
 
 /e2e-playwright/test-plugins/*/dist
 /apps/provisioning/cmd/job-controller/bin/
+
+
+# Ignore unified storage kv store files
+/grafana-kv-data