Feature request: support for `callbacks`

[jnu]

The OpenAPI specification allows of the definition of callback operations. FastAPI supports this by constructing an additional router and passing this router's routes to the relevant path decorator. (More information here.)

This is mostly a problem for documentation, since the callback has no functional meaning to FastAPI. But ideally, the OpenAPI files generated by FastAPI (/openapi.json, /docs/, /redoc/) are as close to the original schema as possible.