Alerting: Fetch alerts from a remote Alertmanager (#75844)

* Alerting: post alerts to the remote Alertmanager and fetch them

* fix broken tests

* Alerting: Add Mimir Backend image to devenv (blocks)

* add alerting as code owner for mimir_backend block

* Alerting: Use Mimir image to run integration tests for the remote Alertmanager

* skip integration test when running all tests

* skipping integration test when no Alertmanager URL is provided

* fix bad host for mimir_backend

* remove basic auth testing until we have an nginx image in our CI

* add integration tests for alerts

* fix tests

* change SendCtx -> Send, add context.Context to Send, fix CI

* add reover() for functions from the Prometheus Alertmanager HTTP client that could panic

* add TODO to implement PutAlerts in a way that mimicks what Prometheus does

* fix log format

pkg/services/ngalert/api/api_alertmanager.go

@@ -149,6 +149,7 @@ func (srv AlertmanagerSrv) RouteGetAMAlertGroups(c *contextmodel.ReqContext) res
 	}
 
 	groups, err := am.GetAlertGroups(
+		c.Req.Context(),
 		c.QueryBoolWithDefault("active", true),
 		c.QueryBoolWithDefault("silenced", true),
 		c.QueryBoolWithDefault("inhibited", true),
@@ -173,6 +174,7 @@ func (srv AlertmanagerSrv) RouteGetAMAlerts(c *contextmodel.ReqContext) response
 	}
 
 	alerts, err := am.GetAlerts(
+		c.Req.Context(),
 		c.QueryBoolWithDefault("active", true),
 		c.QueryBoolWithDefault("silenced", true),
 		c.QueryBoolWithDefault("inhibited", true),