Do not always dump pydantic models to dict

[psychedelicious]

Request:

When using pydantic models for events, they are dumped to dict by fastapi-events.

Motivation

This is convenient but restrictive. A pydantic model is often more useful for a handler than a dict:

• You can call isinstance on it. This is particularly useful for events where one handler callback handles multiple event types.
• The model may have methods or other logic/data that isn't captured by dumping to dict.
• You can get type hints in event handlers.

Proposed API

Add arg payload_schema_dump: bool = True to dispatch.

The default behaviour is unchanged. When this is set to False, handlers are called with the pydantic model as the payload.

If validate_payload is True, we still do the round-trip to re-validate the payload, but skip the final dump.