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>

vendor/github.com/splitio/go-split-commons/v2/synchronizer/interface.go

@@ -0,0 +1,12 @@
+package synchronizer
+
+// Synchronizer interface for syncing data to and from splits servers
+type Synchronizer interface {
+	SyncAll() error
+	SynchronizeSplits(till *int64) error
+	SynchronizeSegment(segmentName string, till *int64) error
+	StartPeriodicFetching()
+	StopPeriodicFetching()
+	StartPeriodicDataRecording()
+	StopPeriodicDataRecording()
+}