Support srcdoc attribute in iframe component.

[AaronNHart]

Do you think it would be reasonable to add srcdoc attribute support to the iframe component? This would enable embedding arbitrary html. The sandbox attribute might go along with this in order to enable scripts.

For context, I have been looking into doing some data visualisation in FastUI but this could also be useful for embedding reports, for example from MultiQC.

# Use it in FastUI like:
c.Iframe(srcdoc='<p>FastUI is neat</p>', src='https://pydantic.dev', width='100%', height=400),

[samuelcolvin]

sounds good to me, PR welcome.