feat(vnets) import existing NAT gateways for cert and trusted networks

[dduportal]

In order to restrict the Azure SAS tokens and Cloudflare R2 tokens to only trusted.ci agents, we need to use the existing NAT gateways for trusted.ci but on the new sponsorship networks to keep outbound IP the same.

Same for cert.ci of course.

Related to jenkins-infra/helpdesk#3875 and jenkins-infra/helpdesk#2649.

This PR imports the existing gateways (removed from terraform management but not deleted in jenkins-infra/azure in jenkins-infra/azure#567)

• Need to (manually) migrate the resources in the vnet RGs instead of controller RGs (it was a mistake I did when setting up these NAT gateways)
• Need to import (manually) the resources

Note: For a subsequent PR: opportunity to create a module to avoid repetition and as such make it easy to spin up NAT gateway for public and private networks.

[dduportal]

No changes. Your infrastructure matches the configuration.

• Migration of RGs + import worked well
• Removed the gateway enablement as peering require routing table: we'll start with a NAT gateway module and multiple public IPs instead (easier)