Add NixOS configuration (closes #102) (#107)

Adds `flake.nixosConfigurations.testnet-validator`, the intitial
configuration that was used for deploying `union-testnet-2`. Will be
expanded later on.

## Commits

- feat(testnet-validator): scaffold nixosConfiguration
- feat(testnet-validator): add `unionvisor` module
- feat(unionvisor): use `union-testnet-2` as the default network
- feat(unionvisor): add union-testnet.cor.systems to seeds
- chore(testnet-validator): cleanup
- docs(testnet-validator): comments for module settings