Review support of AF_UNIX sockets on the Windows port

[wrowe]

Windows introduced AF_UNIX domain socket support to the winsock API with the Insider Build 17063 which dropped in Dec '17. This should now be available mainstream on most deployed Server 2019, although this needs to be validated.

https://devblogs.microsoft.com/commandline/af_unix-comes-to-windows/

In porting this support, Envoy should gracefully degrade where the afunix.sys driver is not installed.

[stale]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or other activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically closed because it has not had activity in the last 37 days. If this issue is still valid, please ping a maintainer and ask them to label it as "help wanted". Thank you for your contributions.

[wrowe]

@snowp this remains a placeholder, could you add help wanted (or we can leave it closed for the time being)

[wrowe]

Note that this is experimentally introduced in #10293 although has yet to be thoroughly reviewed and tested.

[wrowe]

It appears we've made enough progress to consider this placeholder completed.