Add Extreme VOSS - #3742

@ipspace here's my first attempt at VOSS, with very minimal SPB support. I'm very new to how SPB works and have no real-world experience with it, so I don't have a good reference for what a "proper" setup should look like.

This initial support brings up VOSS devices with a working SPB fabric, and netlab adds VLANs, I-SIDs, access/trunk ports, VLAN interface IPs and static routes on top.
I'm not ready to start modelling SPB as a dedicated module, but happy to hear your point of view. For example, IS-IS/SPBM part and the VLAN-to-I-SID mapping should eventually move to this new module/plugin.

Extreme VOSS (Fabric Engine): initial clab support

Adds Extreme VOSS as a minimal-support device, running as vrnetlab containers (tested with vrnetlab/extreme_voss:9.4.0.0 and vrnetlab/extreme_voss:9.3.1.0).

• Device definition (devices/voss.yml): GigabitEthernet 1/N interfaces, clab via vrnetlab with container healthcheck, vlan (switch model) and routing (static, IPv4 global table only).
• Configuration uses community.network.voss_config, the only VOSS Ansible module; it's deprecated. This is documented in the caveats.
• Readiness check: VOSS doesn't run a command passed on the SSH command line (ssh returns 255), so the internal ssh check can't be used. The device uses netlab_ready: [ ansible ] with readiness-check/voss.yml (show sys-info | head 5) instead.
• Initial config: hostname, port names, no shutdown, VLAN interface IPv4, and a minimal SPBM underlay (router isis: area 49.0001, B-VIDs 4051/4052). NNI ports, system ID and nickname are left to the platform's defaults (Auto-Sense / zero-touch fabric).
• VLAN module: VLAN creation, access and trunk ports (with native VLAN). Each VLAN is mapped to an SPB I-SID (10000 + VLAN ID) so it's extended across the fabric.
• Routing module: IPv4 static routes.
• Docs: a VOSS caveat entry and the clab image table entry.