Add User Defined Route optional configuration to point outbound traffic to a hub firewall

[jjgriff93]

We should make it possible to pass some simple config in config.yaml to configure UDRs that can point outbound traffic to route to a peered hub vnet's Firewall/NVA. As part of this we'd need to consider whether the vnet peering should also happen from this repo's deployment process, or whether this should be passed in as a parameter to happen elsewhere.