samples: bluetooth: ipsp: add net config auto init

Add CONFIG_NET_CONFIG_AUTO_INIT=y to enable the bluetooth
device network interface.

Due to #27405 feature PR allowing application to init the network
whenever it wants, IPSP sample stopped initing the
bluetooth network.

The IPSP sample was not updated to enable
CONFIG_NET_CONFIG_AUTO_INIT in prj.conf
and didn't call net_config_init_app(...)
as suggested in the feature PR.

Signed-off-by: Jeremy LOCHE <lochejeremy@gmail.com>