`.json()` unable to serialize datetime elements in `data` payload

Expected Behavior

The CloudEvent model, like Pydantic's BaseModel, can serialize of datetime properties to strings in the .json() function.

Actual Behavior

The CloudEvent model raises a ValueError from the standard library's json.dumps function when .json() is called on a model containing datetime properties in the data field.

Alternatively, if this is intentional behavior, it would be helpful to have some guidance around what the expected usage is when users want to have dates or timestamps in their CloudEvent data.

Steps to Reproduce the Problem

1. Init a CloudEvent where some element of the data argument is a datetime instance.
2. Call .json() on the CloudEvent instance

For example:

from datetime import datetime

from cloudevents.pydantic import CloudEvent

event = CloudEvent(attributes={'source': 'my.source', 'type': 'com.my.type'}, data={'dt': datetime.now()})

# raises TypeError
print(event.json())

By contrast, Pydantic's BaseModel is able to encode datetime properties:

from datetime import datetime

from pydantic import BaseModel

class MyModel(BaseModel):
  dt: datetime

foo = MyModel(dt=datetime.now())

# encodes dt as a string
print(foo.json())

Specifications

• Platform: MacOS Ventura 13.2.1 (Intel)
• Python Version: 3.9.9

The issue identifies cloudevents.pydantic.CloudEvent.json() as the entry point; start by comparing its datetime handling with Pydantic BaseModel.json(). Add a regression case using datetime in data and verify that serialization produces a string rather than raising. Confirm the behavior with the reproduction shown in the issue.