Validate policy mode value

Policy creation succeeds even if the policy mode in the request is
not capitalized, however, when fetching the policy mode value from
Azure, the correct capitalized version is returned, which causes
the Terraform state and configuration to be different.

Ideally, there should be an enum with a set of allowed values, but
this is not the case in the latest version of the Azure Rest API specs.
However, the supported mode values are returned in the error
response when an invalid mode is requested.

Fixes #8748

azurerm/internal/services/policy/policy_definition_resource.go

@@ -63,6 +63,20 @@ func resourceArmPolicyDefinition() *schema.Resource {
 			"mode": {
 				Type:     schema.TypeString,
 				Required: true,
+				ValidateFunc: validation.StringInSlice(
+					[]string{
+						"All",
+						"Indexed",
+						"Microsoft.ContainerService.Data",
+						"Microsoft.CustomerLockbox.Data",
+						"Microsoft.DataCatalog.Data",
+						"Microsoft.KeyVault.Data",
+						"Microsoft.Kubernetes.Data",
+						"Microsoft.MachineLearningServices.Data",
+						"Microsoft.Network.Data",
+						"Microsoft.Synapse.Data",
+					}, false,
+				),
 			},
 
 			"management_group_id": {