Add cross-region replication support to AlloyDB

[modular-magician]

To Create a Secondary Cluster, cluster_type and secondary_config are the additional fields to be mentioned in the cluster.
The secondary cluster is dependent on the primary instance and the same needs to be added in the config using the depends_on field.
The field continuous_backup_config.enabled needs to be set false as continuous backup is not supported for secondary clusters.
The deletion_policy is set to FORCE as the secondary instance that will be created in the secondary cluster can not be deleted independently, but instead the entire secondary cluster needs to be deleted forcefully along with its secondary instance.

  cluster_type = "SECONDARY"
  deletion_policy = "FORCE"
  secondary_config {
    primary_cluster_name = <fully qualified primary cluster name>
    // Eg: primary_cluster_name = google_alloydb_cluster.<primary_cluster_name>.name
  }

  continuous_backup_config {
    enabled = false
  }

  depends_on = [google_alloydb_instance.<primary_instance_name>]

Part of hashicorp/terraform-provider-google#13251

Release Note Template for Downstream PRs (will be copied)

alloydb: added `cluster_type` and `secondary_config` fields to support secondary clusters in `google_alloydb_cluster` resource. 

Derived from GoogleCloudPlatform/magic-modules#9012