feat(pubsub): support global default labels merged with subscription-specific labels

[fbtravi]

Previously, the subscription_labels variable applied mandatory labels to all subscriptions.
This PR introduces per-subscription labels by adding an optional labels field inside each subscription.

• Global labels (subscription_labels) continue to be applied to all subscriptions.
• Subscription-specific labels can now be set and are merged with the global defaults.
• Subscription-specific labels override global labels in case of conflicts.

This change is backwards-compatible and does not break previous configurations.

module "pubsub" {
  source  = "terraform-google-modules/pubsub/google"
  version = "~> 7.0"

  project_id           = var.project_id
  create_topic         = false
  create_subscriptions = true
  topic                = google_pubsub_topic.example.id
  subscription_labels = { env = "dev" }

  pull_subscriptions = [
    {
      name                 = "pull"
      ack_deadline_seconds = 10
      labels = {  feature = "billing"  }
    },
  ]

  push_subscriptions = [
    {
      name                 = "push"
      push_endpoint        = "https://${var.project_id}.appspot.com/"
      x-goog-version       = "v1beta1"
      ack_deadline_seconds = 20
      expiration_policy    = "1209600s" // two weeks
    },
  ]

}

[fbtravi]

Hello, sorry for the new message, but this is very important for the organization I intend to set up.
We will have a project with almost 150 topics, and using the labels this way will help a lot with organization.