Do not use TypeMap for non-primitive types, use TypeList

Currently, TypeMap doesn't support non-primitive type values.
ref : hashicorp/terraform-plugin-sdk#62

When using non-primitive values with TypeMap, terraform crashes at
runtime with a following error message.

```
panic: Unknown validation type: 6
```

This error is originated at
https://github.com/hashicorp/terraform-plugin-sdk/blob/v2.11.0/helper/schema/schema.go#L2017
Only TypeBool, TypeInt, TypeFloat and TypeString are supported.

So, in this change, use TypeList with new custom resource to store
non-primitive values.

.changelog/1520.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+resource/cloudflare_notification_policy: Fix unexpected crashes when using cloudflare_notification_policy with a filters attribute
+```

cloudflare/resource_cloudflare_notification_policy.go

@@ -155,7 +155,7 @@ func buildNotificationPolicy(d *schema.ResourceData) cloudflare.NotificationPoli
 	}
 
 	if filters, ok := d.GetOk("filters"); ok {
-		notificationPolicy.Filters = filters.(map[string][]string)
+		notificationPolicy.Filters = filters.([]interface{})[0].(map[string][]string)
 	}
 
 	if conditions, ok := d.GetOk("conditions"); ok {

cloudflare/schema_cloudflare_notification_policy.go

@@ -25,11 +25,26 @@ func resourceCloudflareNotificationPolicySchema() map[string]*schema.Schema {
 			Required: true,
 		},
 		"filters": {
-			Type:     schema.TypeMap,
+			Type:     schema.TypeList,
 			Optional: true,
-			Elem: &schema.Schema{
-				Type: schema.TypeList,
-				Elem: schema.TypeString,
+			MaxItems: 1,
+			Elem: &schema.Resource{
+				Schema: map[string]*schema.Schema{
+					"zones": {
+						Type: schema.TypeList,
+						Elem: &schema.Schema{
+							Type: schema.TypeString,
+						},
+						Optional: true,
+					},
+					"services": {
+						Type: schema.TypeList,
+						Elem: &schema.Schema{
+							Type: schema.TypeString,
+						},
+						Optional: true,
+					},
+				},
 			},
 		},
 		"created": {