Add a -socket option that configures the server to listen on a Unix-domain socket or Windows named pipe instead of a TCP port. This allows webhook to be used behind a reverse proxy on multi-tenant shared hosting without the need to choose (and the permission to bind to) a free port number.
On Windows, -socket is expected to be a named pipe such as \\.\pipe\webhook, and the code uses https://github.com/microsoft/go-winio to bind the listening socket. On other platforms, -socket is the path to a Unix domain socket such as /tmp/webhook.sock, or an abstract socket name starting with @, bound using the regular net.Listen function with the "network" parameter set to "unix".
Note: this pushes our minimum Go version up to 1.21 as that is what go-winio requires, but that is already the minimum version against which we are testing in the CI matrix.
