Multi-provider topologies

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Wouldn't it be nice if you could run a few nodes in VMs and a few others in containers? That's what I'll try to do with multi-provider topologies. Obviously we'll be limited in what can be done. I can see these easy combos:

• libvirt + containerlab
• anything + external (happily ignoring connectivity issues)

Configuration would be extremely easy: add provider attribute to nodes to specify which provider a node should use. The main provider (top-level topology attribute) would have a list of allowed secondary providers so we'll stop the unsupported combinations.

Using external provider with another one is a piece of cake (conceptually, still needs to be done): call the proper netlab up/down hooks at the proper time and move on.

Combining libvirt and clab providers will be more fun.

netlab create (or equivalent) will have to:

• Set link type to bridge for any mixed-provider segment - libvirt provider hook
• Create Vagrantfile
• Modify the lab topology (remove libvirt nodes, clean up links...) - libvirt provider hook
• Create prototype clab.yml - potentially from within the same libvirt provider hook

netlab up will have to:

• Execute vagrant up
• Collect Linux bridge names from libvirt bridge names and modify the prototype clab.yml file
• Start the containerlab part of the lab (potentially just execute containerlab deploy command)

netlab down will have to:

• Execute containerlab destroy command to bring down the containers
• Execute vagrant destroy -f to bring down the VMs
• Execute cleanup

[ipspace]

Initial implementation in mp-699 branch, documentation in docs/providers.md. Have fun ;)

[ipspace]

Implemented in #706