Fix permadiff in google_bigquery_datapolicy_data_policy policy tag (GoogleCloudPlatform#11722)

Author: wj-chen

mmv1/products/bigquerydatapolicy/DataPolicy.yaml

@@ -82,6 +82,7 @@ properties:
       Policy tag resource name, in the format of projects/{project_number}/locations/{locationId}/taxonomies/{taxonomyId}/policyTags/{policyTag_id}.
     required: true
     diff_suppress_func: 'tpgresource.ProjectNumberDiffSuppress'
+    custom_flatten: 'templates/terraform/custom_flatten/bigquery_datapolicy_policy_tag_location_to_lower_case.go.erb'
   - !ruby/object:Api::Type::Enum
     name: dataPolicyType
     description: |

mmv1/templates/terraform/custom_flatten/bigquery_datapolicy_policy_tag_location_to_lower_case.go.erb

@@ -0,0 +1,31 @@
+<%- # the license inside this block applies to this file
+	# Copyright 2024 Google Inc.
+	# Licensed under the Apache License, Version 2.0 (the "License");
+	# you may not use this file except in compliance with the License.
+	# You may obtain a copy of the License at
+	#
+	#     http://www.apache.org/licenses/LICENSE-2.0
+	#
+	# Unless required by applicable law or agreed to in writing, software
+	# distributed under the License is distributed on an "AS IS" BASIS,
+	# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+	# See the License for the specific language governing permissions and
+	# limitations under the License.
+-%>
+func flatten<%= prefix -%><%= titlelize_property(property) -%>(v interface{}, d *schema.ResourceData, config *transport_tpg.Config) interface{} {
+	if v == nil {
+		return nil
+	}
+
+	if _, ok := v.(string); !ok {
+		return v
+	}
+
+	re := regexp.MustCompile(`(projects/.*)(/locations/.*/)(policyTags/.*)`)
+	result := re.ReplaceAllStringFunc(v.(string), func(match string) string {
+		matches := re.FindStringSubmatch(match)
+		return matches[1] + strings.ToLower(matches[2]) + matches[3]
+	})
+
+	return result
+}