Events: Break out and non-exhaust context body structs (#54)

This PR makes many of the types under `EventContext` separate `#[non_exhaustive]` structs. This makes it more feasible to add further information to connection and packet events as required in future. On this note, driver (re)connection events now include the SSRC supplied by Discord and the domain name which was connected to.

In addition, this fixes global timed events to return a list of all live tracks, and extensively details/documents events at a high level.

This was tested using `cargo make ready`.

src/events/context/data/connect.rs

@@ -0,0 +1,21 @@
+/// Voice connection details gathered at setup/reinstantiation.
+#[derive(Clone, Debug, Eq, Hash, PartialEq)]
+#[non_exhaustive]
+pub struct ConnectData<'a> {
+    /// The domain name of Discord's voice/TURN server.
+    ///
+    /// With the introduction of Discord's automatic voice server selection,
+    /// this is no longer guaranteed to match a server's settings. This field
+    /// may be useful if you need/wish to move your voice connection to a node/shard
+    /// closer to Discord.
+    pub server: &'a str,
+    /// The [RTP SSRC] *("Synchronisation source")* assigned by the voice server
+    /// for the duration of this call.
+    ///
+    /// All packets sent will use this SSRC, which is not related to the sender's User
+    /// ID. These are usually allocated sequentially by Discord, following on from
+    /// a random starting SSRC.
+    ///
+    /// [RTP SSRC]: https://tools.ietf.org/html/rfc3550#section-3
+    pub ssrc: u32,
+}