net: lib: add wifi_credentials library

Upstream NCS's library for storing Wi-Fi credentials.
This library allows storage of Wi-Fi credentials
using different backends.
Either the Zephyr settings subsystem
or the PSA secure backend can be used.
For testing purposes, credentials can be defined statically.

Signed-off-by: Ravi Dondaputi <ravi.dondaputi@nordicsemi.no>
Signed-off-by: Kapil Bhatt <kapil.bhatt@nordicsemi.no>
Signed-off-by: Gregers Gram Rygg <gregers.gram.rygg@nordicsemi.no>
Signed-off-by: Kaja Koren <kaja.koren@nordicsemi.no>
Signed-off-by: Simen S. Røstad <simen.rostad@nordicsemi.no>
Signed-off-by: Maximilian Deubel <maximilian.deubel@nordicsemi.no>