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_test.go

@@ -71,6 +71,7 @@ func TestDesanitize(t *testing.T) {
 	assert.Equal(t, *actual.ElasticsearchSettings.Password, *target.ElasticsearchSettings.Password)
 	assert.Equal(t, actual.SqlSettings.DataSourceReplicas, target.SqlSettings.DataSourceReplicas)
 	assert.Equal(t, actual.SqlSettings.DataSourceSearchReplicas, target.SqlSettings.DataSourceSearchReplicas)
+	assert.Equal(t, actual.ServiceSettings.SplitKey, target.ServiceSettings.SplitKey)
 }
 
 func TestFixInvalidLocales(t *testing.T) {