feat(vnets) add a new subnet for the ci.jenkins.io agents 1 AKS cluster

[dduportal]

This PR adds a new subnet to support the new cluster described in jenkins-infra/helpdesk#3954.

Notes:

• As per the proposed network sizing, the subnet is only a /24 as we don't expected more than ~200 nodes and pods will have their own private CIDR.
  • Note we can grow this subnet for now if needed (vnet is a /14 so we still have plenty of available ranges)
• The NAT gateway is not yet associated: it looks like the custom module does not like specifying a non existing subnet. I've opened feat(gateways) prepare ci.jenkins.io vnet for more agents (AKS) #233 for this when the subnet will be created