subsys/settings: Allow to register custom backends in settings subsystem

Major changes are:
- Expose settings backend API to enable custom backend support.
- Add a new CONFIG_SETTINGS_CUSTOM backend to allow registering a custom
backend.
- Change api of the handlers h_set() routines to allow for
backend-specific read callbacks.
- Provide a customizable settings_backend_init() routine for custom
backends.
- Move runtime settings support to be its own backend.

Signed-off-by: François Delawarde <fnde@oticon.com>