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/generated_base_api_provisioning.go

@@ -23,6 +23,8 @@ type ProvisioningApiForkingService interface {
 	RouteDeleteContactpoints(*models.ReqContext) response.Response
 	RouteGetContactpoints(*models.ReqContext) response.Response
 	RouteGetPolicyTree(*models.ReqContext) response.Response
+	RouteGetTemplate(*models.ReqContext) response.Response
+	RouteGetTemplates(*models.ReqContext) response.Response
 	RoutePostContactpoints(*models.ReqContext) response.Response
 	RoutePostPolicyTree(*models.ReqContext) response.Response
 	RoutePutContactpoints(*models.ReqContext) response.Response
@@ -40,6 +42,14 @@ func (f *ForkedProvisioningApi) RouteGetPolicyTree(ctx *models.ReqContext) respo
 	return f.forkRouteGetPolicyTree(ctx)
 }
 
+func (f *ForkedProvisioningApi) RouteGetTemplate(ctx *models.ReqContext) response.Response {
+	return f.forkRouteGetTemplate(ctx)
+}
+
+func (f *ForkedProvisioningApi) RouteGetTemplates(ctx *models.ReqContext) response.Response {
+	return f.forkRouteGetTemplates(ctx)
+}
+
 func (f *ForkedProvisioningApi) RoutePostContactpoints(ctx *models.ReqContext) response.Response {
 	conf := apimodels.EmbeddedContactPoint{}
 	if err := web.Bind(ctx.Req, &conf); err != nil {
@@ -96,6 +106,26 @@ func (api *API) RegisterProvisioningApiEndpoints(srv ProvisioningApiForkingServi
 				m,
 			),
 		)
+		group.Get(
+			toMacaronPath("/api/provisioning/templates/{ID}"),
+			api.authorize(http.MethodGet, "/api/provisioning/templates/{ID}"),
+			metrics.Instrument(
+				http.MethodGet,
+				"/api/provisioning/templates/{ID}",
+				srv.RouteGetTemplate,
+				m,
+			),
+		)
+		group.Get(
+			toMacaronPath("/api/provisioning/templates"),
+			api.authorize(http.MethodGet, "/api/provisioning/templates"),
+			metrics.Instrument(
+				http.MethodGet,
+				"/api/provisioning/templates",
+				srv.RouteGetTemplates,
+				m,
+			),
+		)
 		group.Post(
 			toMacaronPath("/api/provisioning/contact-points"),
 			api.authorize(http.MethodPost, "/api/provisioning/contact-points"),