Add support for parameter manager regional parameter datasource google_parameter_manager_regional_parameter (#12640) (#20866)

[upstream:9f506b9edc862aeb6c3bf280059c5539e684408c]

Signed-off-by: Modular Magician <magic-modules@google.com>

Author: modular-magician

.changelog/12640.txt

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+```release-note:new-datasource
+`google_parameter_manager_regional_parameter` (beta)
+```

google/services/parametermanagerregional/data_source_parameter_manager_regional_parameter.go

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+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+package parametermanagerregional

google/services/parametermanagerregional/data_source_parameter_manager_regional_parameter_test.go

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+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+package parametermanagerregional_test

website/docs/d/parameter_manager_regional_parameter.html.markdown

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+---
+subcategory: "Parameter Manager"
+description: |-
+  Get information about a Parameter Manager Regional Parameter.
+---
+
+# google_parameter_manager_regional_parameter
+
+Use this data source to get information about a Parameter Manager Regional Parameter.
+
+~> **Warning:** This datasource is in beta, and should be used with the terraform-provider-google-beta provider.
+See [Provider Versions](https://terraform.io/docs/providers/google/guides/provider_versions.html) for more details on beta datasources.
+
+## Example Usage 
+
+```hcl
+data "google_parameter_manager_regional_parameter" "reg_parameter_datasource" {
+  parameter_id = "foobar"
+  location = "us-central1"
+}
+```
+
+## Argument Reference
+
+The following arguments are supported:
+
+* `parameter_id` - (required) The name of the regional parameter.
+
+* `location` - (required) The location of the regional parameter. eg us-central1
+
+* `project` - (optional) The ID of the project in which the resource belongs.
+
+## Attributes Reference
+See [google_parameter_manager_regional_parameter](https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/parameter_manager_regional_parameter) resource for details of all the available attributes.