logging: net: Make autostart of backend configurable

In certain application configuration the networking log backend must not
be started, when the application starts. Starting it must be delayed
until routes to the logging server are established.

This commit introduces a new Kconfig setting to be able to control that
behaviour.

Fixes #27464

Signed-off-by: Christian Taedcke <christian.taedcke@lemonbeat.com>