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.

---------

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

src/config.rs

@@ -34,13 +34,13 @@ pub struct Config {
     #[cfg(feature = "driver")]
     /// Selected tagging mode for voice packet encryption.
     ///
-    /// Defaults to [`CryptoMode::Normal`].
+    /// Defaults to [`CryptoMode::Aes256Gcm`].
     ///
     /// Changes to this field will not immediately apply if the
     /// driver is actively connected, but will apply to subsequent
     /// sessions.
     ///
-    /// [`CryptoMode::Normal`]: CryptoMode::Normal
+    /// [`CryptoMode::Aes256Gcm`]: CryptoMode::Aes256Gcm
     pub crypto_mode: CryptoMode,
 
     #[cfg(all(feature = "driver", feature = "receive"))]
@@ -211,7 +211,7 @@ impl Default for Config {
     fn default() -> Self {
         Self {
             #[cfg(feature = "driver")]
-            crypto_mode: CryptoMode::Normal,
+            crypto_mode: CryptoMode::Aes256Gcm,
             #[cfg(all(feature = "driver", feature = "receive"))]
             decode_mode: DecodeMode::Decrypt,
             #[cfg(all(feature = "driver", feature = "receive"))]