MM-28859 Add feature flag managment system using split.io and remove viper. (#15954)

* Add feature flag managment system using split.io and remove viper.

* Fixing tests.

* Attempt to fix postgres tests.

* Fix watch filepath for advanced logging.

* Review fixes.

* Some error wrapping.

* Remove unessisary store interface.

* Desanitize SplitKey

* Simplify.

* Review feedback.

* Rename split mlog adatper to split logger.

* fsInner

* Style.

* Restore oldcfg test.

* Downgrading non-actionable feature flag errors to warnings.

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

web/web_test.go

@@ -68,10 +68,7 @@ func Setup(tb testing.TB) *TestHelper {
 }
 
 func setupTestHelper(t testing.TB, store store.Store, includeCacheLayer bool) *TestHelper {
-	memoryStore, err := config.NewMemoryStoreWithOptions(&config.MemoryStoreOptions{IgnoreEnvironmentOverrides: true})
-	if err != nil {
-		panic("failed to initialize memory store: " + err.Error())
-	}
+	memoryStore := config.NewTestMemoryStore()
 	*memoryStore.Get().AnnouncementSettings.AdminNoticesEnabled = false
 	*memoryStore.Get().AnnouncementSettings.UserNoticesEnabled = false
 	var options []app.Option