Events: Add SsrcKnown event

Knowing your own SSRC is useful for handling RTCP packets, which may detail information about *ourselves* rather than another host. In theory, at least: this confirms that Discord just sends ReceiverReports containing your own packet stats.

This would have been better to fit into Driver(Re)Connect, but that would be a breaking change: when this change is made, `SsrcKnown` shall be deprecated.

src/driver/connection/mod.rs

@@ -7,6 +7,7 @@ use super::{
 };
 use crate::{
     constants::*,
+    events::CoreContext,
     model::{
         payload::{Identify, Resume, SelectProtocol},
         Event as GatewayEvent,
@@ -195,6 +196,10 @@ impl Connection {
             .mixer
             .send(MixerMessage::SetConn(mix_conn, ready.ssrc))?;
 
+        let _ = interconnect
+            .events
+            .send(EventMessage::FireCoreEvent(CoreContext::SsrcKnown(ssrc)));
+
         spawn(ws_task::runner(
             interconnect.clone(),
             ws_msg_rx,