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>

app/options.go

@@ -52,7 +52,7 @@ func Config(dsn string, watch bool) Option {
 }
 
 // ConfigStore applies the given config store, typically to replace the traditional sources with a memory store for testing.
-func ConfigStore(configStore config.Store) Option {
+func ConfigStore(configStore *config.Store) Option {
 	return func(s *Server) error {
 		s.configStore = configStore