Co-Authored-By: Mahmood Ali <mahmood@hashicorp.com>

Also includes unit tests for binpacker and preemption.
The tests verify that network resources specified at the
task group level are properly accounted for

Co-Authored-By: Michael Schurter <michael.schurter@gmail.com>

Also DRY up Networks code by using Networks.Copy

Adds support for passing network isolation config into drivers and
implements support in the rawexec driver as a proof of concept

Adds a new Prerun and Postrun hooks to manage set up of network namespaces
on linux. Work still needs to be done to make the code platform agnostic and
support Docker style network initalization.

Co-Authored-By: nickethier <ncethier@gmail.com>