feature request: support modifying datagram to allow customizable protocol header

[oyyd]

Firstly, thanks for the great repository!

Currently, tokio_kcp can't communicate with kcp-go because: beside kcp, kcp-go also adds other features that change its protocol header, e.g. crypto, crc, fec. It should be emphasized that these features are kcp-go specific.

Though, being compatible with kcp-go might be not the goal of tokio_kcp, I believe adding the ability for users to modify the datagram to support their own headers makes it possbile. It would also extend the potential usage of tokio_kcp.