list: update code example for list to set identity data (#1025)

Currently this code example works without issues, but a recent
validation change in framework means that this will begin to return an
error since it isn't valid to return a list result with an identity
containing all null values.

hashicorp/terraform-plugin-framework#1230

Author: stephybun

content/terraform-plugin-sdk/v2.38.x/docs/plugin/sdkv2/resources/list.mdx

@@ -96,6 +96,17 @@ func (r *ThingListResource) List(ctx context.Context, req list.ListRequest, stre
 			// Set the ID of the resource for the ResourceData object
 			rd.SetId("a_unique_id_related_to_the_thing")
 
+			// Get the existing identity data from ResourceData and set relevant identity attributes
+			identity, err := rd.Identity()
+			if err != nil {
+				result.Diagnostics.AddError(
+                    "Retrieving identity data",
+                    "An error was encountered when retrieving the identity data: "+err.Error(),
+                )
+			}
+
+			identity.Set("an_identity_attribute", thing.IdentityAttribute)
+
 			// Set the attributes of interest into the resource state
 			rd.Set("name", thing.Name)
 			rd.Set("attribute_one", thing.AttributeOne)