Alerting: Send alerts to external Alertmanager(s) (#37298)

* Alerting: Send alerts to external Alertmanager(s)

Within this PR we're adding support for registering or unregistering
sending to a set of external alertmanagers. A few of the things that are
going are:

- Introduce a new table to hold "admin" (either org or global)
  configuration we can change at runtime.
- A new periodic check that polls for this configuration and adjusts the
  "senders" accordingly.
- Introduces a new concept of "senders" that are responsible for
  shipping the alerts to the external Alertmanager(s). In a nutshell,
this is the Prometheus notifier (the one in charge of sending the alert)
mapped to a multi-tenant map.

There are a few code movements here and there but those are minor, I
tried to keep things intact as much as possible so that we could have an
easier diff.

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

@@ -0,0 +1,51 @@
+package definitions
+
+// swagger:route GET /api/v1/ngalert/admin_config configuration RouteGetNGalertConfig
+//
+//  Get the NGalert configuration of the user's organization, returns 404 if no configuration is present.
+//
+//     Produces:
+//     - application/json
+//
+//     Responses:
+//		 200: GettableNGalertConfig
+//		 404: Failure
+//		 500: Failure
+
+// swagger:route POST /api/v1/ngalert/admin_config configuration RoutePostNGalertConfig
+//
+// Creates or updates the NGalert configuration of the user's organization.
+//
+//     Consumes:
+//     - application/json
+//
+//     Responses:
+//       201: Ack
+//       400: ValidationError
+
+// swagger:route DELETE /api/v1/ngalert/admin_config configuration RouteDeleteNGalertConfig
+//
+// Deletes the NGalert configuration of the user's organization.
+//
+//     Consumes:
+//     - application/json
+//
+//     Responses:
+//       200: Ack
+//       500: Failure
+
+// swagger:parameters RoutePostNGalertConfig
+type NGalertConfig struct {
+	// in:body
+	Body PostableNGalertConfig
+}
+
+// swagger:model
+type PostableNGalertConfig struct {
+	Alertmanagers []string `json:"alertmanagers"`
+}
+
+// swagger:model
+type GettableNGalertConfig struct {
+	Alertmanagers []string `json:"alertmanagers"`
+}