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/api.json

@@ -1281,6 +1281,14 @@
    "title": "Frames is a slice of Frame pointers.",
    "type": "array"
   },
+  "GenericPublicError": {
+   "properties": {
+    "body": {
+     "$ref": "#/definitions/PublicError"
+    }
+   },
+   "type": "object"
+  },
   "GettableAlertmanagers": {
    "properties": {
     "data": {
@@ -4405,6 +4413,7 @@
    "type": "object"
   },
   "alertGroup": {
+   "description": "AlertGroup alert group",
    "properties": {
     "alerts": {
      "description": "alerts",
@@ -4533,7 +4542,6 @@
    "type": "object"
   },
   "gettableAlert": {
-   "description": "GettableAlert gettable alert",
    "properties": {
     "annotations": {
      "$ref": "#/definitions/labelSet"
@@ -5641,6 +5649,12 @@
     "responses": {
      "204": {
       "description": " The mute timing was deleted successfully."
+     },
+     "409": {
+      "description": "GenericPublicError",
+      "schema": {
+       "$ref": "#/definitions/GenericPublicError"
+      }
      }
     },
     "summary": "Delete a mute timing.",