[MM-28692] Include config diffs in audit record for config changing API calls (#17623)

* Replace config generator

* Cleanup

* Some renaming and docs additions to add clarity

* Cleanup logging related methods

* Cleanup emitter

* Fix TestDefaultsGenerator

* Move feature flags synchronization logic out of config package

* Remove unnecessary util functions

* Simplify load/set logic

* Refine semantics and add some test to cover them

* Remove unnecessary deep copies

* Improve logic further

* Fix license header

* Review file store tests

* Fix test

* Fix test

* Avoid additional write during initialization

* More consistent naming

* Update app/feature_flags.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

* Make ConfigStore.Set() return both old and new configs

* Implement config diff function

* Make app.SaveConfig return previous and current configs

* Add config diff to audit record

* Fix returned configs

* Include high level test

* Move FF synchronizer to its own package

* Remove unidiomatic use of sync.Once

* Add some comments

* Rename function

* More comment

* Save config diff in audit record for local endpoints

* Enable audit for config set/reset commands

* Improve tests output

Co-authored-by: Christopher Speller <crspeller@gmail.com>
Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

app/plugin_api.go

@@ -109,7 +109,8 @@ func (api *PluginAPI) GetUnsanitizedConfig() *model.Config {
 }
 
 func (api *PluginAPI) SaveConfig(config *model.Config) *model.AppError {
-	return api.app.SaveConfig(config, true)
+	_, _, err := api.app.SaveConfig(config, true)
+	return err
 }
 
 func (api *PluginAPI) GetPluginConfig() map[string]interface{} {
@@ -123,7 +124,8 @@ func (api *PluginAPI) GetPluginConfig() map[string]interface{} {
 func (api *PluginAPI) SavePluginConfig(pluginConfig map[string]interface{}) *model.AppError {
 	cfg := api.app.GetSanitizedConfig()
 	cfg.PluginSettings.Plugins[api.manifest.Id] = pluginConfig
-	return api.app.SaveConfig(cfg, true)
+	_, _, err := api.app.SaveConfig(cfg, true)
+	return err
 }
 
 func (api *PluginAPI) GetBundlePath() (string, error) {