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/export_test.go

@@ -4,8 +4,6 @@
 package config
 
 import (
-	"io"
-
 	"github.com/jmoiron/sqlx"
 	"github.com/mattermost/mattermost-server/v5/model"
 )
@@ -15,11 +13,6 @@ func MarshalConfig(cfg *model.Config) ([]byte, error) {
 	return marshalConfig(cfg)
 }
 
-// UnmarshalConfig exposes the internal unmarshalConfig to tests only.
-func UnmarshalConfig(r io.Reader, allowEnvironmentOverrides bool) (*model.Config, map[string]interface{}, error) {
-	return unmarshalConfig(r, allowEnvironmentOverrides)
-}
-
 // InitializeConfigurationsTable exposes the internal initializeConfigurationsTable to test only.
 func InitializeConfigurationsTable(db *sqlx.DB) error {
 	return initializeConfigurationsTable(db)