feat(netplan): render InfiniBand (IPoIB) interfaces

[ArtsiomAntropau]

The netplan renderer's type dispatch only handled physical, bond, bridge and vlan interfaces, so type: infiniband links were silently dropped from the generated /etc/netplan/50-cloud-init.yaml. On OpenStack instances (network_data.json -> network config v1) IPoIB interfaces disappeared entirely, even though the eni, sysconfig and network-manager renderers already rendered them.
Add an infiniband branch that configures IPoIB devices under ethernets (netplan has no dedicated infiniband section), matching on the 20-byte IPoIB hardware address and setting infiniband-mode: datagram when an MTU is present, mirroring the NetworkManager renderer. This is possible now that netplan supports IPoIB -- matching the long-form HWADDR and the infiniband-mode key (netplan 0.105, PR canonical/netplan#283) -- which was the missing-netplan-support blocker that had LP: #1848471 (GH #3474) closed as won't-fix.

Proposed Commit Message

feat(netplan): render InfiniBand (IPoIB) interfaces

The netplan renderer's type dispatch only handled physical, bond,
bridge and vlan, so `type: infiniband` interfaces were silently
dropped from the generated netplan. The eni, sysconfig and
network-manager renderers already render them, so on netplan-default
distros IPoIB links from an OpenStack network_data.json (network
config v1) ended up missing entirely.

Render InfiniBand devices under `ethernets` (netplan has no dedicated
infiniband section), matching on the 20-byte IPoIB hardware address
and setting `infiniband-mode: datagram` when an MTU is present, as the
NetworkManager renderer does. Address and MTU handling reuses
`_extract_addresses`.

This was previously not possible because netplan could not match the
long-form IPoIB HWADDR; that support was added in netplan 0.105
(canonical/netplan#283), which unblocks it here.

Fixes GH-3474

Additional Context

The netplan renderer never handled type: infiniband. The interface
loop in Renderer._render_content only has branches for
physical/bond/bridge/vlan, so IPoIB interfaces were silently dropped
from the generated netplan, while the eni, sysconfig and
network-manager renderers already rendered them.

This was reported before (GH-3474, LP: #1848471) and closed won't-fix at
the time, because netplan itself couldn't match the 20-byte IPoIB
HWADDR. That blocker is gone: netplan added IPoIB support in 0.105
(canonical/netplan#283), including matching the long HWADDR and the
infiniband-mode key, so the renderer can finally emit it.

Practical impact: OpenStack instances with IPoIB links
(network_data.json -> network config v1) get those interfaces missing
from netplan on netplan-default distros; only the ENI renderer
configured them.

The new branch renders IB under ethernets (netplan has no dedicated
infiniband section), matches on the HWADDR, and sets
infiniband-mode: datagram when an MTU is present, mirroring the
NetworkManager renderer. Address/MTU handling reuses _extract_addresses.

Test Steps

Unit tests: added a netplan round-trip case and updated the existing
large_v1 fixture, which already contained an IB interface that used to
be dropped.

pytest tests/unittests/test_net.py -k infiniband

Manual check on a live system, non-destructive via net-convert:

1. A v1 config with an IB interface (net.yaml):

    version: 1
    config:
      - type: infiniband
        name: ib0
        mac_address: "00:00:10:48:fe:80:00:00:00:00:00:00:a0:88:c2:03:00:f7:f0:ea"
        mtu: 2044
        subnets:
          - type: static
            address: 192.168.2.80/24

2. Render it:

    cloud-init devel net-convert \
      --network-data net.yaml --kind yaml \
      --distro ubuntu --output-kind netplan --directory out
    cat out/etc/netplan/50-cloud-init.yaml

3. Before this change ib0 is absent. With it:

    network:
      version: 2
      ethernets:
        ib0:
          match:
            macaddress: "00:00:10:48:fe:80:00:00:00:00:00:00:a0:88:c2:03:00:f7:f0:ea"
          addresses:
          - "192.168.2.80/24"
          set-name: ib0
          mtu: 2044
          infiniband-mode: datagram

4. On a host with netplan >= 0.105, netplan generate accepts the output.

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)