Consdier mirroring leap seconds file with GHA

[trey-stafford]

Consider mirroring the leap seconds data file on GH, using scheduled GHA to keep it updated at some interval.

This may require some additional code to check the validity of the file at runtime (i.e.., if a user adjusts a GPS time for a datetime after the last valid datetime in the file, raise an error).

[trey-stafford]

gps-timemachine already stores a static copy of the leap seconds file that it falls back on if reading from the remote source fails! See: https://github.com/nsidc/gps-timemachine/blob/main/gps_timemachine/static/tai-utc.dat

There does not appear to be any code to check the validity of that file, as described above, but it hasn't changed since we added it back in Dec. 2020.

[trey-stafford]

See #4