box, datetime: messagepack support for datetime

Serialize `struct datetime` as newly introduced MP_EXT type.
It saves 1 required integer field and 3 optional fields:
- epoch is required field;
- but nsec, tzoffset and tzindex are optional;

* supported json, yaml serialization formats, lua output mode;

Please refer to the
https://hackmd.io/@Mons/S1Vfc_axK#%D0%A3%D0%BF%D0%B0%D0%BA%D0%BE%D0%B2%D0%BA%D0%B0-%D0%B2-msgpack
for a description of a messagepack serialization schema for datetime
values.

Follow-up to #5941
Part of #5946