use async fn in traits in SendStream

[Ruben2424]

at the moment there are two trait functions to send data.

• send_data to prepare data for sending
• poll_ready to poll the stream until the data is sent

This PR uses the async fn in traits feature to introduce the async fn send_data function in the SendStream trait.

Advantages:

• Simplifying the h3_quinn crate
  • Removes error paths. h3_quinn panics if some functions are called while the boxed future is not ready. Users of h3_quinn+ h3 might be able to get this, for example when using RequestStream in a Mutex. (but I'm not sure)
  • Avoids boxing the future
  • Avoids cloning the data before sending
• In h3 it is no longer possible to accidentally call send_data without poll_ready

Disadvantages

• Breaks MSRV
• Send bounds? (maybe they can be removed).
• idk maybe more?

Open:

• Try to remove Send bounds
• Fix WebTransport
• Fix missing Tests
• Documentation

Is it worth exploring this approach?