Metadata datetime handling.

Relates to https://code.usgs.gov/ghsc/geomag/geomag-plots/-/issues/338

Currently, (at least some) metadata for readings in the metadata service include times without a timezone. In algorithms, using obspy UTCDateTime, these times are interpreted as UTC. In javascript, these times are interpreted as local time.

• Check pydantic formatting for UTCDateTime, it should use UTC and the "Z" timezone abbreviation for unambiguous interpretation everywhere.
• Need to coordinate with geomag-plots/operations app to ensure this does not break current implementation.