Specify datetime json encoder for CliveBaseModel

This issue comes from pydantic ignoring json
encoders in nested structures (does not apply them
recursively):
https://github.com/pydantic/pydantic/issues/2277

Fix for this will be available in pydantic v2.

At first I assumed schemas are serializing datetime incorrectly,
but it turns out that Clive is wrapping models from schemas inside
JSONRPCProtocol model, which inherits from CliveBaseModel.
That's why we need a json encoder defined once again in Clive.