Add support for custom payloads and channels in broadcasting

[JanneDeVos]

This pull request introduces support for broadcasting events to multiple channels and connections with specific payloads for each connection. The changes include:

BroadcastEvent Class Enhancements:

• Added methods to handle broadcasting to multiple connections and channels.
• Implemented logic to format and send payloads specific to each connection.

Test Cases:

• Added new test cases in BroadcastEventTest to verify the functionality of broadcasting to multiple channels and connections.
• Ensured existing tests are updated to reflect the new broadcasting logic.

[bram-pkg]

This is a breaking change!!! @taylorotwell

Say I have a connection named apn and my event emits a payload of

return [
  'apn' => [
    'name' => 'Bram',
  ],
];

After this update, my event will actually only emit [ 'name' => 'Bram' ] rather than the whole payload.

Basically this change disallows for broadcasting any key that matches the current connection name, which is definitely a breaking change.