Refactor constellation broadcast channel (#38077)

- Move the 2 hash maps used to manage channels in their own struct.
- The constellation is still in charge of origin checks since it holds
the pipeline information required.
- BroadcastMsg is renamed to BroadcastChannelMsg for consistency.


Testing: covered by existing tests.
Fixes: #38060

Signed-off-by: webbeef <me@webbeef.org>

components/constellation/lib.rs

@@ -7,6 +7,7 @@
 #[macro_use]
 mod tracing;
 
+mod broadcastchannel;
 mod browsingcontext;
 mod constellation;
 mod constellation_webview;