Control plane support for attached subnets.

[rcgoodfellow]

Omicron support for attaching external and VPC subnets directly to instances. See RFD 599 for more details.

Depends on

• Dendrite support for attached subnets dendrite#163
• OPTE support for external subnets opte#890

Omicron scope of work includes.

• API for attaching external subnets to instances
  • Modify instance subnet_name parameter to be an object with a direct attach option and manage transit_ips accordingly
  • Update VPC subnet allocation/assignment machinery to account for attached VPC subnets
  • Subnet pools
  • Saga/RPW machinery for managing:
    • External subnet routes on the switch
    • External subnet tables on OPTE
    • BGP announcements/withdraws for attached/detached external subnets (operator opt-in)
    • VPC system routes for attached VPC subnets
• Dynamic metadata service