Add binding for agent forwarding

[Riatre]

channel_open_request_auth_agent_callback is required for implementing ssh agent forward as unlike X11 forward, there is no other way to establish a forwarding channel.

In libssh:

1. Callback is triggered while handling protocol packets in other libssh call.
2. The callback creates a new channel and prepare for bidirectional forwarding between it and ssh agent.
3. The callback then returns a borrow of the newly created channel for libssh to make reply to the remote side.

However, the callback-based flow does not really fit our Rust binding design: during callback we have SessionHolder locked, so it's really hard to do anything without introducing lock re-entrancy issues, plus that it demands us to return a temporary borrow of something owned by Rust side whose lifetime is tricky to model.

Instead, we try to turn the callback-based style back to something resembling ssh_channel_accept_x11 by buffering pending channels and let users fetch them later in a saner context.

[Riatre]

I have a draft for using this to implement ssh agent forward in wezterm: wez/wezterm#5345

[wez]

Thanks for this! I'd love to be able to use agent forwarding with wezterm!
I think there are some smallish tweaks that are needed to make this safe

[Riatre]

Thanks for the review! I've addressed most comments, but have further comments on the safety discussion.

In addition, I wonder why do we need std::panic::catch_unwind in bridge_*_callback-s? I just followed suit here.

Edit: nomicon mentioned:

If you are writing Rust code that may panic, and you don't wish to abort the process if it panics, you must use catch_unwind

But we don't have UB here even if we don't catch it: panic will be turned into abort in extern "C" functions. My question holds: do we need to eat the panic and return error code instead of bring down the entire process?

Edit 2: nvm, c_unwind is not in stable yet.

[wez]

Thank you!

[wez]

Published to crates.io as 0.3.1

[tomtastic]

I'd also love to see agent forwarding in WezTerm, but small side-note that the Windows OpenSSH agent (and specifically ssh-add) is broken currently for users of SSH certificates.