[Feature] Simplify the custom event definition.

[wenjin272]

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• I searched in the issues and found nothing similar.

Description

Currently, if user want to send/receive custom events, they must define a new event type, like

class MyEvent(Event):
    """User custom event."""
    field1: str
    field2: int

This can be somewhat cumbersome at times.

To simplify the definition of Agent, we want to support user only declare the identifier, like:
ctx.send_event(identifier="MyEvent", field1="test", field2=1)

Are you willing to submit a PR?

• I'm willing to submit a PR!