Enable ssh agent authentication on Windows

This MR:

• Enables compilation of SSH agent authentication on Windows systems
• Adds support for using IdentityAgent to specify the agent unix domain socket path

The Windows agent auth change is currently running on a number of Windows systems at my place of work.

I haven't yet tested the IdentityAgent changes in earnest. I'd be happy to augment tests for those if you wouldn't mind pointing me in the right direction. I'd also be happy to split this into two separate MRs if you'd prefer.

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code