Add an optional "type" discriminator on each tracking client, selecting
the sender. It defaults to "elasticsearch" so existing configurations keep
working unchanged.

The new "http" sender POSTs the raw tracking JSON to any HTTP collector
(configurable method and headers), so the probe can feed backends other
than Elasticsearch: a Vector/Alloy pipeline forwarding to Grafana Loki
(LGTM), an OpenTelemetry Collector, or a custom endpoint.

Related to #14