net: conn_mgr: Support Auto-Connect

To reduce the need for boilerplate in application code, conn_mgr now
supports an auto-connect feature on all ifaces with connectivity
bindings.

conn_mgr will automatically call conn_mgr_if_connect on any iface with a
connectivity binding that enters the admin-up state, unless the newly
added CONN_MGR_IF_NO_AUTO_CONNECT flag has been set for that iface.

Also adjust automated tests to account for and take advantage of this
behavior.

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