feat: v8 encryption modes (#264)

This PR adds support for the new AEAD cryptosystems advertised by Discord, AES256-GCM and XChaCha20Poly1305. These schemes will shortly become mandatory, and provider stronger integrity/authentication guarantees over the cleartext portions of any voice packet by correctly specifying additional authenticated data.

To provide smooth switchover, we've added basic negotiation over the `CryptoMode`. This ensures that any clients who are manually specifying one of the legacy modes will automatically migrate to `Aes256Gcm` when Discord cease to advertise their original preference.

Closes #246.

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Co-authored-by: Kyle Simpson <kyleandrew.simpson@gmail.com>

benches/mixing-task.rs

@@ -31,8 +31,6 @@ use songbird::{
 };
 use std::{io::Cursor, net::UdpSocket, sync::Arc};
 use tokio::runtime::{Handle, Runtime};
-use xsalsa20poly1305::{KeyInit, XSalsa20Poly1305 as Cipher, KEY_SIZE};
-
 fn no_passthrough(c: &mut Criterion) {
     let rt = Runtime::new().unwrap();