Add support reading IdentityAgent from OpenSSH configuration.

[dkocher]

Resolves #10454.

[sindastra]

Hi there, are these changes already live? I'm asking because I downloaded Cyberduck just now (version 8.2.3), and this does not work for me.

[lnhrdt]

Hey @dkocher I'm really excited for this feature. I've been wishing this type of OpenSSH configuration was supported for years, so thank you!

I saw in the release notes this was included with 8.2.2 but I'm using 8.2.3 and unable to figure out how to make it work. Is there anything special we need to do in the connection configuration, or otherwise? Or are there any limitations to what configurations are supported?

[florisvdg]

Same for me, haven't been able to get this to work. Launching the app with SSH_AUTH_SOCK does work (already prior to this PR), but anything I try with IdentityAgent does not.

@dkocher Do you have an example config snippet that works for you?

[dkocher]

Possibly related to #12939.