Allow setting hash algorithm to use for signing requests of SSH agent

[wiktor-k]

This is a suggestion PR that I've tested locally. I'm happy to adjust it in any way you see fit.

@nightmared would it be possible for you to test it, too? (just like in the ticket you've described).

Fixes: #444
Fixes: #445

Note that since I'm using my own agent server implementation and upstream ssh-key has not released a version with some critical fixes such as RustCrypto/SSH#263 making signatures with SHA-1 is totally broken :( so I'd be really happy if something like this has been merged. Thanks for your time! 🙇

[Eugeny]

I think a more flexible solution would be to update the Signer trait to accept an Option<HashAlg> parameter, similar to how PrivateKeyWithHashAlg is used for direct key auth. If you have time to implement this change, please do, otherwise I'll merge it as-is and do it later, thanks!

It will need to be updated later anyway to accept an enum of a public key OR certificate later for OpenSSH certificate keys agent auth.

[wiktor-k]

Thanks for the feedback! I'll try to implement your idea tomorrow and see how far I go...

[nightmared]

This PR does fix the test (and would close #444)!

However, I'm not convinced hardcoding SHA512 (instead of leaving the option to specify the algorithm as @Eugeny suggested) is the best choice here. E.g., what would happen when talking to old systems/SSH servers that may not support it? I fear this could introduce regressions.

[Eugeny]

(sorry for the conflict, client.rs got moved from russh-keys/src to russh/src/keys on main)

[wiktor-k]

(sorry for the conflict, client.rs got moved from russh-keys/src to russh/src/keys on main)

Fortunately git took care of that seamlessly when rebasing 😅

Okay, I've passed the hash_alg throughout the code and verified that this PR (still) works with my other code... phew. (Note that I've adjusted only signing with the agent, the thing that I use, maybe some other parts need adjusting, too? 🤔 )

See if that's more like what you meant and if not I can re-adjust.

Thanks for your time! 👋

[Eugeny]

LGTM, thank you!