net: app: Split code for configuring network to a separate lib, "config"

net_app library consisted of 2 disjoint parts: a) library to
setup/configure networking on the application startup; b) library
to setup client and server connections.

As the configuration library is universally useful and is a generic
Zephyr networking component, split it out to a seperate top-level
networking library under net/lib/config/.

Fixes: #7658

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>