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>

Adds a network stanza and additional options to the task group level
in prep for allowing shared networking between tasks of an alloc.

command/agent: allow the job GC interval to be configured

This addresses the issue raised by @pznamensky in #5966.