[MP] Add optional authentication to SceneMultiplayer.

Add few methods to allow peers to exchange authentication information.

- `set_auth_callback(callback)`: Enable the authentication features.
  The callback is a `Callable` that accepts an `int` (the peer ID), and
  a `PackedByteArray` of data.
- The `peer_authenticating(id)` signal will be emitted instead of
  `peer_connected` when a new peer connects.
- Use `send_auth(id: int, data: PackedByteArray)` to exchange data.
- Call `complete_auth(id: int)` when the authentication process is
  complete and you expect to start receiving game data.
- The `peer_connected` signal will be emitted as soon as both parties
  complete the authentication.
- Use `disconnect_peer(id)` to disconnect a connected peer.
- If the `peer_connected` signal didn't fire for that peer (i.e. it was
  still in the authentication phase), the `peer_auth_failed` signal will
  be emitted instead of `peer_disconnected`.

0 files changed, 0 insertions, 0 deletions