Standardize timezone of datetime responses

[ml-evs]

Currently we return timezone specific date times, but the spec mandates RFC 3339 (i.e. with "Z" "2007-04-05T14:30Z").

Spec definition:

https://github.com/Materials-Consortia/OPTIMADE/blob/develop/optimade.rst#last-modified

This will probably require tweaking the pydantic Config of the particular datetime properties.

[fekad]

There is a tiny mistake in the specification: the second is missing from the example which is required for the RFC3339 format. If I understand to have a "precise" definition was the main reason to switch from ISO 8601 (Materials-Consortia/OPTIMADE#145)

About the implementation:

• Do you want to define the timezone as a variable in the CONFIG or
• Do you want to use the computer's timezone settings or
• Do you want always to use UTC (with Z" at the end)?

As far as I can see eg. COD always shows UTC...

[CasperWA]

In discussion, we'll go for the third option, always using UTC.