feat: Support defining tags per subnet name

This commit updates the module to support defining subnet tags per subnet name, in addition to per AZ.

Reasoning behind this is to allow for specific tags to be applied to different public/private subnets.
E.g.
```terraform
  private_subnet_names = ["Private Subnet One", "Private Subnet Two"]
  private_subnet_tags_per_name = {
    "Private Subnet One" = {
      "subnet one" = "true"
    },
    "Private Subnet Two" = {
      "subnet two" = "true"
    }
  }
```

Author: fatmcgav