Allow for import format of only numbers in iam folder

[slevenick]

Fixes: hashicorp/terraform-provider-google#4971

google_folder import already behaves this way.

Release Note Template for Downstream PRs (will be copied)

`google_folder`: added support for importing in the form of the bare folder id, rather than requiring `folders/{bare_id}`

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
I see that this PR has already had some downstream PRs generated. Any open downstreams are already updated to your most recent commit, 6bfc4f9.

Pull request statuses

WARNING: The following files changed in commit 6bfc4f9 may need corresponding changes in third_party/validator:

• third_party/terraform/utils/iam_folder.go

No diff detected in terraform-google-conversion.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I built this PR into one or more new PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#1430
depends: hashicorp/terraform-provider-google#4981