[Silabs] Refactor SiWx917 Wi-Fi Application implementation

[mkardous-silabs]

Description

PR transitions the SiWx917 Wi-Fi implementation to use the sl_net API. This allows us to leverage the LwIP integration of the Wi-Fi SDK instead of using the one implemented for Matter for the SiWx917.

• Replace sl_wifi APIs by the sl_net equivalent
• Remove the join callback handler since the sl_net_up API does not trigger the callback and that it is not necessary anymore
• Remove the scan network callback since it always failed due to string length issues. Since we assume the scan can fail and still try to connect, the scan brings little value.

Tests

Siw917x build, flash and commission