Alerting: Add provisioning GET routes for message templates (#48367)

* Template service

* Add GET routes and implement them

* Generate mock for persist layer

* Unit tests for reading templates

* Set up composition root and get integration tests working

* Fix prealloc issue

* Extract setup boilerplate

* Update AuthorizationTest

* Rebase and resolve

* Fix linter error

pkg/services/ngalert/api/api.go

@@ -78,6 +78,7 @@ type API struct {
 	AccessControl        accesscontrol.AccessControl
 	Policies             *provisioning.NotificationPolicyService
 	ContactPointService  *provisioning.ContactPointService
+	Templates            *provisioning.TemplateService
 }
 
 // RegisterAPIEndpoints registers API handlers
@@ -136,6 +137,7 @@ func (api *API) RegisterAPIEndpoints(m *metrics.API) {
 			log:                 logger,
 			policies:            api.Policies,
 			contactPointService: api.ContactPointService,
+			templates:           api.Templates,
 		}), m)
 	}
 }