Add VPC subnet realization data source

This is an alias to segment realization data source, but with
naming that is consistent with VPC.

Signed-off-by: Anna Khmelnitsky <akhmelnitsky@vmware.com>

nsxt/provider.go

@@ -340,6 +340,7 @@ func Provider() *schema.Provider {
 			"nsxt_vpc_ip_address_allocation":                         dataSourceNsxtVpcIpAddressAllocation(),
 			"nsxt_policy_gateway_connection":                         dataSourceNsxtPolicyGatewayConnection(),
 			"nsxt_policy_distributed_vlan_connection":                dataSourceNsxtPolicyDistributedVlanConnection(),
+			"nsxt_vpc_subnet_realization":                            dataSourceNsxtPolicySegmentRealization(),
 		},
 
 		ResourcesMap: map[string]*schema.Resource{

website/docs/d/vpc_subnet_realization.html.markdown

@@ -0,0 +1,52 @@
+---
+subcategory: "VPC"
+layout: "nsxt"
+page_title: "NSXT: vpc_subnet_realization"
+description: State of VPC subnet realization on hypervisors.
+---
+
+# nsxt_vpc_subnet_realization
+
+This data source provides information about the realization of VPC subnet or hypervisor.
+This data source will wait until realization is complete with either success, partial success or error. 
+It is recommended to use this data source in conjunction with vsphere provider, in order to ensure subnet is realized 
+on hypervisor before VM is created on same network.
+
+This data source is applicable to NSX Policy Manager.
+
+## Example Usage
+
+```hcl
+resource "nsxt_vpc_subnet" "s1" {
+  context {
+    project_id = data.nsxt_policy_project.demoproj.id
+    vpc_id     = data.nsxt_vpc.demovpc.id
+  }
+
+  display_name     = "test-subnet"
+  description      = "Test VPC subnet"
+  ipv4_subnet_size = 32
+  ip_addresses     = ["192.168.240.0/24"]
+  access_mode      = "Isolated"
+}
+
+data "nsxt_vpc_subnet_realization" "s1" {
+  path = nsxt_vpc_subnet.s1.path
+}
+
+# usage in vsphere provider
+data "vsphere_network" "net" {
+  name          = nsxt_vpc_subnet_realization.s1.network_name
+  datacenter_id = data.vsphere_datacenter.datacenter.id
+}
+
+## Argument Reference
+
+* `path` - (Required) The policy path of the subnet.
+
+## Attributes Reference
+
+In addition to arguments listed above, the following attributes are exported:
+
+* `state` - The realization state of the resource: `success`, `partial_success`, `orphaned`, `failed` or `error`.
+* `network_name` - Network name on the hypervisor. This attribute can be used in vsphere provider in order to ensure implicit dependency on segment realization.