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:

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

  mqtt {
    broker_url     = "tcp://localhost:1883"
    client_id      = "grafana"
    topic          = "grafana/alerts"
    message_format = "json"
    username       = "user"
    password       = "password123"
    qos            = 1
    retain         = true
    tls_config {
      insecure_skip_verify = true
      ca_certificate       = "ca_cert"
      client_certificate   = "client_cert"
      client_key           = "client_key"
    }
  }
}

[github-actions]

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