feat: Add unix socket support

[mkg20001]

Adds support for requests over unix sockets, similar to how npm.im/request does it
Example: fetch('http://unix:/var/lib/lxd/unix.socket:/') does a GET / on /var/lib/lxd/unix.socket

[bitinn]

Thx for the PR but we have no plan to support unix socket yet.

However, check if this approach allow you to use unix socket through passing in an Agent.

#336 (comment)

[elmerbulthuis]

Would love to see unix socket upport! Also, why not?

Our particular use case is communication with the docker engine. We developed an openapi client that is using node-fetch. One of the clients we generate is for the docker engine. This engine usually listens to a unix socket.