Alerting: time interval service to support addressing intervals by Base64 encoded name (#90563)

* rename to getMuteTimingByName

* add UID to api model of MuteTiming

* update GetMuteTiming to search by UID

* update UpdateMuteTiming to support search by UID

* update DeleteMuteTiming to support uid

* make sure UID is populated

* update usages

* use base64 url-safe, no padding encoding for UID

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

@@ -117,6 +117,7 @@ type MuteTimingHeaders struct {
 
 // swagger:model
 type MuteTimeInterval struct {
+	UID                     string `json:"-" yaml:"-"`
 	config.MuteTimeInterval `json:",inline" yaml:",inline"`
 	Version                 string     `json:"version,omitempty"`
 	Provenance              Provenance `json:"provenance,omitempty"`