drivers: wifi: options for application usage of Wi-Fi

The `CONFIG_WIFI` symbol currently covers a number of different
use-cases of Wi-Fi, primarily:
  * Wi-Fi AP (Broadcasting a network others can connect to)
  * Wi-Fi Station (Connecting to an AP + SSID scanning)
  * Wi-Fi AP + Station (Both the above)
  * Wi-Fi SSID scanning (SSID scanning only)

Each of these application usages of Wi-Fi requires a different set of
supporting options to operate correctly (`WPA_SUPP`,
`NET_CONNECTION_MANAGER`, etc), and different opportunities for resource
optimisation (System heap, buffer counts, etc).

Currently, optimizing the Wi-Fi drivers depends on driver specific
options, which can be challenging to find and are obviously not portable
across different drivers.

The addition of this choice allows applications to specify in a single
Kconfig option which features of Wi-Fi they use, and enable individual
Wi-Fi drivers to choose defaults based on those features.

Signed-off-by: Jordan Yates <jordan@embeint.com>