Feature request: Procedure for adding a member without uplink network connection

OVN and MicroCloud itself supports a heterogenous configuration of cluster members, where
- some members have the `ovn-chassis` role, and are candidates for for the OVN virtual router), while
- other members lack the `ovn-chasses` role, and will not be candidates.

From https://documentation.ubuntu.com/lxd/latest/reference/ovn-internals/#chassis:

> A chassis is where traffic physically ingresses into or egresses out of the virtual network. In LXD, there will usually be one chassis per cluster member. If LXD is configured to use OVN networking, then all members can be used as OVN chassis.
> If any cluster members have the role `ovn-chassis`, only those members are represented as chassis in the chassis group table (see below). If no members have the role, all cluster members are added to the chassis group.

It would be helpful if the `microcloud add` questionnaire also supported adding members without an uplink connection.
(Currently, adding an uplink-less member to a cluster with distributed networking is considered an error: https://github.com/canonical/microcloud/pull/1034#discussion_r2445725568).

Or, subsidiarily, if there was a documented procedure for adding uplink-less members.

Related discussion: https://discourse.ubuntu.com/t/error-binary-attribute-size-out-of-range-when-adding-a-node-without-uplink-nic/70122/5