Add proper support for ST0603 / MISP Time System

[bradh]

Describe the bug
MISP is big on time, but we're being a bit loose around some of the concepts. In particular, we need to be more careful about conversion to and from UTC.

To Reproduce
Steps to reproduce the behavior:

1. Review MISB Handbook and ST0603
2. Note the part about not including leap seconds.

Expected behavior
Probably move the existing code (plus code we should have) in ST0601 for parsing out timestamps, correction factors and leap seconds into some shared code. Make sure that the types (LocalDateTime, ZonedDateTime) we're using underneath have the right leap second semantics.
Rework the ST0903 times to use that.

Maybe add support for the nanosecond timestamp.

Screenshots
N/A

Configuration (please complete the following information):
N/A

Additional context
I'll try to get this done, but it looks like it could be more complex that I originally thought, so there is no schedule.

Definition of Done (subject to change):

[X] Migrate shared code to new st0603 module (covered by #114 )
[X] Add time status implementation (covered by #114)
[ ] Update API / docs / implementation to reflect MISP vs UTC references
[ ] Add conversion routines that handle different time bases (basically the 12 cases in Motion Imagery Handbook 2020.1 Table 6-4.)
[ ] Use conversion routines in ST1909 to show FT and MT in UTC.

out of scope:

• Nanosecond time stamp conversion. This can be a separate ticket if we need it.