Settings: Encapsulate settings within an extensible provider (#32219)

* Encapsulate settings with a provider with support for runtime reloads

* SettingsProvider: reload is controlled by the services

* naive impl of reload handlers for settings

* working naive detection on new changes

* Trigger settings reload from API endpoint

* validation step added

* validation of settings

* Fix linting errors

* Replace DB_Varchar by DB_NVarchar

* Reduce settings columns (section, key) lenghts

* wip db update logic

* Db Settings: separate updates and removals

* Fix: removes incorrectly added code

* Minor code improvements

* Runtime settings: moved oss -> ee

* Remove no longer used setting.Cfg SAML-related fields

* Rename file setting/settings.go => setting/provider.go

* Apply suggestions from code review

Co-authored-by: Agnès Toulet <35176601+AgnesToulet@users.noreply.github.com>

* Minor code improvements on OSS settings provider

* Fix some login API tests

* Correct some GoDoc comments

* Apply suggestions from code review

Co-authored-by: Agnès Toulet <35176601+AgnesToulet@users.noreply.github.com>

Co-authored-by: Leonard Gram <leo@xlson.com>
Co-authored-by: Agnès Toulet <35176601+AgnesToulet@users.noreply.github.com>

pkg/api/http_server.go

@@ -73,6 +73,7 @@ type HTTPServer struct {
 	Bus                    bus.Bus                                 `inject:""`
 	RenderService          rendering.Service                       `inject:""`
 	Cfg                    *setting.Cfg                            `inject:""`
+	SettingsProvider       setting.Provider                        `inject:""`
 	HooksService           *hooks.HooksService                     `inject:""`
 	CacheService           *localcache.CacheService                `inject:""`
 	DatasourceCache        datasources.CacheService                `inject:""`