Add read/write functions accepting additional authentication data to StatelessTransportState

[pr2502]

Hi,

I'm implementing a Noise-based protocol which needs to add a plaintext header to each message but have it still authenticated. This required only exposing the encrypt_ad and decrypt_ad functions which the private CipherState type already implements.

This is an interface other Noise libraries like https://pkg.go.dev/github.com/flynn/noise#CipherState.Decrypt or https://docs.rs/noise-protocol/latest/noise_protocol/struct.CipherState.html#method.encrypt_ad already expose so I hope this is an ok addition to snow?