feat: Support defining tags per subnet name

[fatmcgav]

Description

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

Motivation and context

Reasoning behind this is to allow for specific tags to be applied to different public/private subnets.
E.g.

  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"
    }
  }

Fixes #1129

[fatmcgav]

Doh, that'll teach me to not check open PR's first... #1131 does essentially the same thing...