Add new exporter to push JSON to a raw TCP socket

[lingfish]

This is for example useful with Graylog.

Although Graylog has various HTTP inputs, they are for GELF and others, which would need additional formatters on the rspamd side.

Using the raw input results in IO timeouts from the rspamd side, because Graylog's raw input doesn't "speak" HTTP.

This resolves the issue. Combined with an appropriate Graylog pipeline to split into fields, this works well.

Example Graylog pipeline:

rule "JSON to fields"
when
  starts_with(to_string($message.message), "{") && ends_with(to_string($message.message), "}")
then
  let json = parse_json(to_string($message.message));
  let map = to_map(json);
  set_fields(map, "rspamd_");
  set_field("rspamd_score", to_double(map.score));
end