net: conn_mgr: Bulk convenience functions

To further reduce the need for networking boilerplate in applications,
provide bulk versions of net_if_up, net_if_down, conn_mgr_if_connect,
and conn_mgr_if_disconnect that affect all available / eligible ifaces
at once.

Since it is not intuitive whether these functions should affect ifaces
which conn_mgr is ignoring, these functions take an argument that allows
this to be specified by the application.

Signed-off-by: Georges Oates_Larsen <georges.larsen@nordicsemi.no>