[rust-netstack] Add SocketProvider FIDL interface

This adds a stub of the fuchsia.net.SocketProvider interface, which
simply returns ENOSYS for all calls to Socket. Functionality will be
added in a later CL.

Test: Manually run.
Change-Id: Ib82bdd81b03896b14a1c80f18bc645b8bc3db92c
3 files changed