add support for leap second smearing

[t3lurid3]

Summary: This adds leap second smearing for fblock library users that want to get Truetime in UTC and not TAI. This is part of a stack. This change updates the C lib and can land when the sidecar changes are deployed. RFC here.

~~This introduces ~18 hour linear smear thats starts at the leap second event time (to closely align with production NTP).

• 24 hour duration smear is the most widely used (specifics of implementations vary) but a shorter smear period is more appropriate to our production environment so as to more closely align with NTP synchronized UTC timestamps.
• Smearing starts at the leap second event time.
• Supports positive and negative leap seconds (i.e. can slow down or speed up UTC time).
• the smearing is applied to phctime_ns and then WOU is
• The smear is a step function (1ns smear per 65us during clock smear period)
• Uses a fixed offset (if tzdatais not read in by fblock sidecar) as we haven't open sourced that.

Reviewed By: abulimov

Differential Revision: D56827880

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D56827880

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D56827880

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D56827880

[facebook-github-bot]

This pull request has been merged in facebook/time@67cca8cd17d6f5ab83fabde40f827b8fa46ffa2d.