Enable alternate ssh agents for step ssh on Windows

[drbrowen]

pipe. If present, use the pipe specified instead of the default. Allows interoperability with other ssh agents, such as pageant.

Name of feature:

Enable alternate ssh agents on Windows

Pain or issue this feature alleviates:

This fixes issue #1185 This parses windows openssh .ssh/config file to look for an alternate IdentityAgent and attempts to use this pipe instead of the default.

Why is this important to the project (if not answered above):

This enables alternate Windows Identity agents, such as Pageant, which eases integration into PuTTY and winscp

In what environments or workflows is this feature supported?

This enhances the Windows Openssh features.

Supporting links/other PRs/issues:

💔Thank you!