[gigaboot] Add managed network ethernet agent

Add an ergonomic wrapper around the managed network protocol in the
form of an EthernetAgent.
The agent is responsible for
*) Finding a valid network interface
*) Constructing the layer 2 header
*) Transmitting a payload

The agent is not yet used by any other code, but it is intended to be
used by the mDNS broadcast rewrite.

Change-Id: Ib7d1fff1146c6cb2a457dd8b624ae08b79943214
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/886653
Reviewed-by: Sergii Parubochyi <sergiip@google.com>
Commit-Queue: Dov Shlachter <dovs@google.com>
Reviewed-by: David Pursell <dpursell@google.com>