Resource `coderd_group`. Allow Importing Groups Created by OIDC

Currently, there's no way to import groups created by an OIDC provider as a `coderd_group` per this [snippet](https://github.com/coder/terraform-provider-coderd/blob/d0761cc89fe8c00b5c6f7445099f81ae0e3733a9/internal/provider/group_resource.go#L369).

The OIDC provider is configured to auto-create the group if it doesn't exist. But, we also have most of our groups are managed by the Coderd provider. It would be nice to allow it to be managed by Terraform after it's initial creation from the OIDC provider alongside the rest of our groups. Is it possible to have this done?