cmd/derpprobe,prober: add ability to perform continuous queuing delay measurements against DERP servers

This new type of probe sends DERP packets sized similarly to CallMeMaybe packets
at a rate of 10 packets per second. It records the round-trip times in a Prometheus
histogram. It also keeps track of how many packets are dropped. Packets that fail to
arrive within 5 seconds are considered dropped.

Updates tailscale/corp#24522

Signed-off-by: Percy Wegmann <percy@tailscale.com>
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