add alert heartbeats

[satta]

One can now use the heartbeat.alert-times list to specify when an alert heartbeat should be injected.

heartbeat:
  enable: true
  # 24h HH:MM strings with local times to send heartbeat as HTTP event
  times:
    - "00:01"
  # 24h HH:MM strings with local times to send heartbeat as alert
  alert-times:
    - "00:02"

The approach is identical to the one for the general heartbeats: at each specified time, an alert like

{
    "timestamp": "2021-12-09T09:49:35.641252+0000",
    "event_type": "alert",
    "src_ip": "192.0.2.1",
    "src_port": 39106,
    "dest_ip": "192.0.2.2",
    "dest_port": 80,
    "proto": "TCP",
    "alert": {
        "action": "allowed",
        "gid": 0,
        "signature_id": 0,
        "rev": 0,
        "signature": "DCSO FEVER TEST alert",
        "category": "Not Suspicious Traffic",
        "severity": 0
    },
    "http": {
        "hostname": "test-2021-12-09.vast",
        "url": "/just-visiting",
        "http_user_agent": "FEVER",
        "http_content_type": "text/html",
        "http_method": "GET",
        "protocol": "HTTP/1.1",
        "status": 200,
        "length": 42
    }
}

is generated and injected in to the stream of forwarded events.