Add JSON validation to the aws_iam_policy resource (#10239)

Although the aws_iam_policy has a problem of normalization (refs #8350),
I think it would be useful simply to add JSON syntax validation.
I wasted a lot of time with JSON syntax errors.

Validate the aws_iam_policy using the validateJsonString helper.

builtin/providers/aws/resource_aws_iam_policy.go

@@ -34,6 +34,7 @@ func resourceAwsIamPolicy() *schema.Resource {
 			"policy": &schema.Schema{
 				Type:         schema.TypeString,
 				Required:     true,
+				ValidateFunc: validateJsonString,
 			},
 			"name": &schema.Schema{
 				Type:          schema.TypeString,