Add SSH agent client with an example

[wiktor-k]

I'm not happy with the API (especially the Client requiring type for the ListeningSocket) but overall this works so I'm sharing for any potential feedback :)

[jcspencer]

Looks great! I've added a few comments around the error handling (it may be good to expose a clearer error to the consumer, rather than returning UnexpectedResponse), but otherwise this looks great! The example is super clear as well, nice work!

[wiktor-k]

Great comments about the errors! As I mentioned in the other comment I think this is a bigger problem and we should be migrating away from the Box into maybe a more specific error type... I'll check it out in another PR but I consider it mostly orthogonal to this PR (as it applies to all others too :( ).

[jcspencer]

Looks great to me! The API here (mirroring the agent-side Session) is very clean!

[wiktor-k]

Looks great to me! The API here (mirroring the agent-side Session) is very clean!

Haha, thanks! 😊 It occurred to me that thanks to the trait being the same one can build and agent that simply forwards requests to another agent with relative ease :)

I'm going to merge it now to avoid API drift even though I had a couple of tweaks in mind.