Alerting: MuteTiming service return errutil + GetTiming by name (#79772)

* add get mute timing by name to MuteTimingService
* update get mute timing request handler to use the service method

* replace validation, uniqueness and used errors with errutils
* update mute timing methods return errutil responses
* use the term "time interval" in errors bevause mute timings are deprecated in Alertmanager and will be replaced by time intervals in the future.

* update create and update methods to return struct instead of pointer

pkg/services/ngalert/api/tooling/definitions/shared.go

@@ -20,3 +20,10 @@ type ForbiddenError struct {
 	// in: body
 	Body errutil.PublicError `json:"body"`
 }
+
+// swagger:model
+type GenericPublicError struct {
+	// The response message
+	// in: body
+	Body errutil.PublicError `json:"body"`
+}