Add BigQuery subcriptions to Pubsub module.

Pubsub can now have subscriptions that write directly to BigQuery.
 * https://cloud.google.com/pubsub/docs/bigquery

In the Google Terraform provider, this is configured using an additional block
inside a `google_pubsub_subscription` resource.
 * https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/pubsub_subscription#nested_bigquery_config

This PR adds a new input variable to the `pubsub` module, to optionally add this
block to some of the subscriptions defined in the module.

modules/pubsub/main.tf

@@ -116,6 +116,16 @@ resource "google_pubsub_subscription" "default" {
       }
     }
   }
+
+  dynamic "bigquery_config" {
+    for_each = try(var.bigquery_subscription_configs[each.key], null) == null ? [] : [""]
+    content {
+      table               = var.bigquery_subscription_configs[each.key].table
+      use_topic_schema    = var.bigquery_subscription_configs[each.key].use_topic_schema
+      write_metadata      = var.bigquery_subscription_configs[each.key].write_metadata
+      drop_unknown_fields = var.bigquery_subscription_configs[each.key].drop_unknown_fields
+    }
+  }
 }
 
 resource "google_pubsub_subscription_iam_binding" "default" {