Addional Time Servers

[Rabenherz112]

• ntpd-rs (https://github.com/pendulum-project/ntpd-rs) - NTP and NTS
• LinuxPTP Project - PTP
• Maybe "NTP Reference Implementation" (ntpd) - NTP

[Technetium1]

I don't feel like the reference implementation deserves to be included, never really see it used. Indirectly related PR #728