Release 3.1.0

• Add optional vsock and tokio-vsock features, allowing connections to be easily created for an AF_VSOCK socket. While none of the existing brokers support this (yet), it's already usable in two cases:
  • p2p
  • Firecracker's vsock support, where the host side is a normal UNIX socket and hence a D-Bus broker can be bound on that and a zbus-based app or service in the guest can connect through this new vsock support.
• Generalize Sink impl of Connection to T: Into<Arc<Message>>, to make it more consistent with the MessageStream API and to allow users to simply connect a MessageStream to Connection (as Sink) and have the sink consume the stream.
• ObjectServer now correctly handles NO_REPLY_EXPECTED flag on incoming method calls.
• Auth code improvements and fixes:
  • Add support for pipelined SASL handshakes, improving sd-bus compatibility.
  • Handle EXTERNAL data in a separate message.
  • Allow empty data with EXTERNAL.
  • Require data with ANONYMOUS.