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/split_logger.go

@@ -0,0 +1,35 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package config
+
+import (
+	"fmt"
+
+	"github.com/mattermost/mattermost-server/v5/mlog"
+)
+
+type splitLogger struct {
+	wrappedLog *mlog.Logger
+}
+
+func (s *splitLogger) Error(msg ...interface{}) {
+	s.wrappedLog.Error(fmt.Sprint(msg...))
+}
+
+func (s *splitLogger) Warning(msg ...interface{}) {
+	s.wrappedLog.Warn(fmt.Sprint(msg...))
+}
+
+// Ignoring more verbose messages from split
+func (s *splitLogger) Info(msg ...interface{}) {
+	//s.wrappedLog.Info(fmt.Sprint(msg...))
+}
+
+func (s *splitLogger) Debug(msg ...interface{}) {
+	//s.wrappedLog.Debug(fmt.Sprint(msg...))
+}
+
+func (s *splitLogger) Verbose(msg ...interface{}) {
+	//s.wrappedLog.Info(fmt.Sprint(msg...))
+}