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/setting/setting.go

@@ -297,10 +297,6 @@ type Cfg struct {
 	// OAuth
 	OAuthCookieMaxAge int
 
-	// SAML Auth
-	SAMLEnabled             bool
-	SAMLSingleLogoutEnabled bool
-
 	// JWT Auth
 	JWTAuthEnabled       bool
 	JWTAuthHeaderName    string
@@ -1175,10 +1171,6 @@ func readAuthSettings(iniFile *ini.File, cfg *Cfg) (err error) {
 	SigV4AuthEnabled = auth.Key("sigv4_auth_enabled").MustBool(false)
 	cfg.SigV4AuthEnabled = SigV4AuthEnabled
 
-	// SAML auth
-	cfg.SAMLEnabled = iniFile.Section("auth.saml").Key("enabled").MustBool(false)
-	cfg.SAMLSingleLogoutEnabled = iniFile.Section("auth.saml").Key("single_logout").MustBool(false)
-
 	// anonymous access
 	AnonymousEnabled = iniFile.Section("auth.anonymous").Key("enabled").MustBool(false)
 	cfg.AnonymousEnabled = AnonymousEnabled