Alerting: Add MQTT notifications receiver

[alexander-akhmetov]

Adds support for MQTT notifications receiver.

• PR to Grafana: https://github.com/grafana/grafana/pull/91487
• PR to alerting: https://github.com/grafana/alerting/pull/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
  }
}