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>

config/utils.go

@@ -76,6 +76,10 @@ func desanitize(actual, target *model.Config) {
 	if target.ServiceSettings.GfycatApiSecret != nil && *target.ServiceSettings.GfycatApiSecret == model.FAKE_SETTING {
 		*target.ServiceSettings.GfycatApiSecret = *actual.ServiceSettings.GfycatApiSecret
 	}
+
+	if *target.ServiceSettings.SplitKey == model.FAKE_SETTING {
+		*target.ServiceSettings.SplitKey = *actual.ServiceSettings.SplitKey
+	}
 }
 
 // fixConfig patches invalid or missing data in the configuration, returning true if changed.