Support MSYS2 ssh-agent sockets on Windows

[wheybags]

Summary

Support using ssh keys from msys installations, like the one which ships with git, or anything else which can connect to an ssh-agent socket.

Context

I use ssh and git on windows through the copy of msys that ships with git (git bash).
I can start an ssh-agent there, or I can set the SSH_AUTH_SOCKET environment variable.
When I used keeagent, I could set a socket location in keeagent settings, and it would run an ssh agent that I could point at in git bash.
I can't currently (to the best of my knowledge) use keepassxc's ssh agent functionality from git bash.

I can see two methods of implementing this:

• Do what keeagent does, and implement an ssh agent server which can be accessed through a configurable path. This is preferable to me, as it is the easiest to set up automatically on login (just put an export SSH_AUTH_SOCKET=blah line in my .bashrc, and configure the same path in keepassxc)
• Allow keepassxc to use some configurable path to a socket file on windows like it does on linux/osx. AFAIK, on linux/osx keepassxc will add its keys to an agent that it connects to via a file specified by the SSH_AUTH_SOCKET environment variable. A similar method could be used on windows, either through the same standard environment variable, or by some config setting. This would require the user to work somemagic to ensure the environment variable was set before starting keepassxc, but is more in line with standard practice on other systems.

Apologies if this is a duplicate issue, I did try searching older issues, but I didn't find anything.

[droidmonkey]

Will be in the next release: #3801

[wheybags]

Ah great, thanks!

[hifi]

Overriding the socket is only supported on non-Windows platforms in develop at this point.

Does MSYS2 use a named pipe like OpenSSH for Windows does?