Add support for creating subfolders using terraform

[suntala]

Subtask of the Nested Folders epic. This is not a GA feature. It is only supported if the nestedFolders feature flag is enabled.

• Make sure tests are running: https://github.com/grafana/terraform-provider-grafana/blob/master/internal/resources/grafana/resource_folder_test.go . We'll need to add a resource with parent folder here because we'll need it for the next step.
• Add a test to make sure the terraform provider is working with subfolders e.g. https://github.com/grafana/grafana/blob/d3546558f3c89e80706a48ee3c496b5dd64fa4d7/pkg/services/folder/folderimpl/folder_test.go#L719
• Update the terraform provider docs: https://github.com/grafana/terraform-provider-grafana/blob/master/docs/resources/folder.md. Shows up with changes here
• Test the changes to the terraform provider in a grafana cloud dev instance. We can create a dev instance (todo: find guide/use pre-existing stack). Maybe there's a way to create the whole stack through terraform. Enable the feature flag by overriding the configuration (guide).

Acceptance Criteria

• Able to create folder with subfolder via terraform resource in a dev instance.

Slack context

[danielthedm]

@danielthedm to sync with the as-code squad to inform them of the new generated go client work. Also to sync on who can complete the terraform provider work to support the nested folders work.