rdmacm: expose connection private

[perbu]

I'm evaluating sideway as a replacement for a hand-written rdma-core C shim in
an HTTP-over-RDMA transport (I promise it isn't as crazy as it sounds), and it covers almost the whole
surface cleanly, thanks for the crate. One capability I couldn't reach is RDMA-CM private data, and I wanted to suggest exposing it. This
is so I can get (version / capabilities / credentials) and such.

Maybe something like this?

impl ConnectionParameter {
    pub fn setup_private_data(&mut self, data: &[u8]) -> &mut Self;
}

impl Event {
    pub fn private_data(&self) -> &[u8];
}

Is this something you wanna consider adding? I think it should be in rdma-mummy-sys so it is mostly about exposing it.