feat(serverless_negs): add serverless_deployment support for API Gateway

[litzius]

Description

Adds serverless-deployment as a new valid type in serverless_neg_backends, enabling the serverless_deployment block on google_compute_region_network_endpoint_group resources.

This is useful for routing traffic to API Gateway backends via an external HTTP(S) load balancer.

Changes

• autogen/main.tf.tmpl / modules/serverless_negs/main.tf: add dynamic "serverless_deployment" block
• autogen/variables.tf.tmpl / modules/serverless_negs/variables.tf: extend service object with optional platform and url_mask fields; add serverless-deployment to validation

Usage example

serverless_neg_backends = [{
  region = "us-central1"
  type   = "serverless-deployment"
  service = {
    name     = "my-gateway-id"
    platform = "apigateway.googleapis.com"
  }
}]