Note that this PR is continuing the work started on #68127. Most of the commits come directly from that PR, couple of latest ones are new as they add settings db support.

Executive summary of how the PR works:

• create yaml file describing the network configuration
• at build time, create a const C struct from this yaml data and compile it to application
• at runtime, allow user to tweak the configuration and store changes to settings db
• at boot, apply the default and user changed configuration to network subsystem

More detailed description follows:

This PR adds support to autoconfigure the network sub-system when the device boots. It is mainly meant to be used when there are multiple network interfaces in the system. In that case Kconfig based network configure parameters cannot be used easily as the system will only apply the configuration to one of the network interfaces.

This is related to old enhancement #29750 request. There it was pondered to use devicetree to configuring network stack. As DT is only meant to describe hardware configuration some other way of configuration is needed.

This PR uses yaml file to describe network stack configuration. The configuration data in yaml file is not restricted to network configuration but any subsystem configuration could be done via it. I concentrated to network configuration as we need this multi network interface autoconfiguration in networking.

The PR provides minimal infrastructure to validate a user supplied yaml file and then generate a C configuration data file from it. So we do not import the yaml file directly to the zephyr image, but we use a generator script to generate C header file that is then included by the network config library. When the device boots, the network configuration library (subsys/net/lib/config/init.c) reads the configuration stored in flash, and applies the configuration to relevant network interface. When the user creates the yaml file in host system, they should know the network configuration in order to map the yaml file data to the device network configuration. The configuration data from flash is used as a default read-only configuration.

The user supplied yaml file can be stored outside of zephyr. When building the network application, the location of the yaml file is given so the build system can then utilize it.

At runtime, user is able to adjust the configuration values stored in flash. The changes will be stored in settings database, the backend can be selected by the user. The changes to the network configuration can be done via net_config_get/set() API or via a net config shell which provides commands (view/set/remove) to adjust the values in the network configuration. When the device boots, the default network configuration in flash is used as a base and any user changed network configuration values from settings db are applied on top of the default network configuration.