Alerting: Add MQTT notifications receiver

[alexander-akhmetov]

Adds support for MQTT notifications receiver.

• PR to Grafana: Alerting: Add MQTT notifications receiver grafana#91487
• PR to alerting: Add MQTT receiver alerting#227

Example:

variable "grafana_mqtt_password" {
  type = string
}

resource "grafana_contact_point" "mqtt" {
  name = "mqtt-contact-point"

  mqtt {
    broker_url           = "ssl://127.0.0.1:8883"
    topic                = "grafana/alerts"
    insecure_skip_verify = false
    username             = "grafana"
    password             = var.grafana_mqtt_password
  }
}

[github-actions]

In order to lower resource usage and have a faster runtime, PRs will not run Cloud tests automatically.
To do so, a Grafana Labs employee must trigger the cloud acceptance tests workflow manually.

[yuri-tceretian]

should it have testutils.CheckOSSTestsEnabled(t, ">=11.3.0")?