- Add helpers to check if an address provided by a user can be
  requested, i.e., is in the subnet and is not a reserved address.
- Add a query used to insert a network interface into the database,
  handling both IP allocation and validation of any provided instance
  (i.e., that the instance spans exactly one VPC). This subsumes the
  previous IP allocation query.
- Removes the `instance_create_network_interface` method, moving the
  instance create saga to use the new
  `vpc_subnet_create_network_interface` method, with an instance
  provided in the construction request.
- Adds parameters for setting up network interfaces (possibly multiple)
  in the instance create request.
- Adds support for specifying zero or more network interfaces at
  instance creation. These must be new interfaces, support for attaching
  to existing interfaces is not yet implemented.