Add a human-readable name to platforms

This would not necessarily be used by Conch itself, but could be useful to downstream clients to more easily communicate with their users.

I'd imagine the config looking like:
```toml
[platforms."batch.cluster1.example"]
name = "Cluster 1"  # ← this
alias = "cluster1.example"
hostname = "1.access.example.com"
proxy_jump = "bastion.example.com"
```

and would be returned in the `/sign` response.