Fix: allow user to specify datetime and UUID objects in post content

[odesenfans]

Problem: using create_post() with an object containing datetime, UUIDs and other types not serializable by the Python json module fails.

Solution: as we already use Pydantic all over the place, use the Pydantic encoder as default for all the types not serializable by the json module.

[hoh]

Looks like a decent idea :+1: