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.
For 3rd parties building binary packages, and for build consistency in
general, it is very helpful to have the same set of dependencies at
any time the product is built.
See [tools/godep](https://github.com/tools/godep) for further details.
