Introduce RoughtimeClient

Roughtime is Google's new network time protocol[1] and we'd like to
start using it for some devices in place of/in addition to NTP. This
patch gets the beginnings of an implementation off the ground, with tests.

[1] https://roughtime.googlesource.com/roughtime/

Test: New unit tests pass
Bug: 30834326
Change-Id: I401d8ad59c82dda514fd3c4be63804f46814251d