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>

cmd/mattermost/commands/plugin_test.go

@@ -37,17 +37,21 @@ func TestPlugin(t *testing.T) {
 
 	fs, err := config.NewFileStore(th.ConfigPath(), false)
 	require.Nil(t, err)
-	require.NotNil(t, fs.Get().PluginSettings.PluginStates["testplugin"])
-	assert.True(t, fs.Get().PluginSettings.PluginStates["testplugin"].Enable)
-	fs.Close()
+	cfsStore, err := config.NewStoreFromBacking(fs)
+	require.Nil(t, err)
+	require.NotNil(t, cfsStore.Get().PluginSettings.PluginStates["testplugin"])
+	assert.True(t, cfsStore.Get().PluginSettings.PluginStates["testplugin"].Enable)
+	cfsStore.Close()
 
 	output = th.CheckCommand(t, "plugin", "disable", "testplugin")
 	assert.Contains(t, output, "Disabled plugin: testplugin")
 	fs, err = config.NewFileStore(th.ConfigPath(), false)
 	require.Nil(t, err)
-	require.NotNil(t, fs.Get().PluginSettings.PluginStates["testplugin"])
-	assert.False(t, fs.Get().PluginSettings.PluginStates["testplugin"].Enable)
-	fs.Close()
+	cfsStore, err = config.NewStoreFromBacking(fs)
+	require.Nil(t, err)
+	require.NotNil(t, cfsStore.Get().PluginSettings.PluginStates["testplugin"])
+	assert.False(t, cfsStore.Get().PluginSettings.PluginStates["testplugin"].Enable)
+	cfsStore.Close()
 
 	th.CheckCommand(t, "plugin", "list")