feat(vpc): add support for ResourceIdentity

[yfodil]

Import can be done by identity now:

import {
  to       = scaleway_vpc.main
  identity = {
    id     = "11111111-1111-1111-1111-111111111111"
    region = "fr-par"
  }
}


resource "scaleway_vpc" "main" {}

[remyleone]

Could you flatten more and resolve early in case :

• identity is nil,
• identity does not contain id and resolve with a specific diagnostic
• identity does not contain region and resolve with a specific diagnostic