Logging: Add Kconfig template for log inheriting

Add a new Kconfig template that allow log modules to inherit their log
level from their parent module.
For example, the logs used in the Bluetooth audio like
`BT_AUDIO_STREAM_LOG_LEVEL` can inherit their level from
`BT_AUDIO_LOG_LEVEL`.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>