Network: Add PTP clock synchronization protocol

[Ryanf55]

Feature request

Is your feature request related to a problem? Please describe.

The MAVLink TIMESYNC Protocol V2 is not precise enough for some applications, and it's bespoke. It also doesn't guarantee sync with the GPS, which is instead estimated using Jitter Correction. I'd like ArduPilot to support a clock synchronization protocol that is standardized and supported by nice GNSS units such as the Trimble PX-1. This allows the GNSS System, ArduPilot, and the companion computer, and even some cameras/sensors to all use the same clock.

I propose PTP per RFC-8173 and IEEE 1588-2008. This should achieve 10uS precision.

Describe the solution you'd like

• Use the LWIP network support to add PTP support to ardupilot being a PTP slave. Assume the GPS is the PTP master.
• Use the LWIP network stack to have Ardupilot be the PTP master when you don't have GPS

Describe alternatives you've considered

• MAVLINK timesync
• PPS
• NTP

Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

Additional context

https://discuss.ardupilot.org/t/mavlink-time-synchronization/128496/4