Summary of the new feature/enhancement

Support using "sockaddr_un" (AF_UNIX sockets) with Invoke-RestMethod.

Proposed technical implementation details

Example usage:
`Invoke-RestMethod sockaddr_un://var/run/docker.sock/containers/json

As far as I understand the socket layer should behave the same so no big changes in the code, except adding the scheme and address family option. I don't think this should affect Powershell code.
System.Uri will need to support new scheme, which obviously means changes in framework & core.