Add extended JSON encoder for datetime objects in message encoding

[MHHukiewitz]

Supersedes aleph-im/aleph-client#131

[github-actions]

• Addition of a new function extended_json_encoder which handles serialization for various datetime types. This is a significant change as it affects the way data is encoded and decoded in the application.
• Modification of the _prepare_aleph_message function to use the new extended_json_encoder instead of the previous encoder pydantic_encoder. This could potentially introduce bugs if the new encoder does not handle all cases that the previous encoder handled.

[1yam]

LGTM