net: conn_mgr: generic wifi_mgmt connectivity backend

As connectivity backends need to be bound in the same file that the
`net_if` is created in, define a common backend type for WiFi modems.
All WiFi modems should be controllable through the `wifi_mgmt` API, so
there should be no need for dedicated context.

When enabled, the particular implementation to be used is chosen through
`CONNECTIVITY_WIFI_MGMT_IMPL`. For now, the only choice is an
application defined backend.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>