Terraform: support pinning tags to digests

### Describe the proposed change(s).

Support pinning from:

```
source = "git@github.com:terraform-aws-modules/terraform-aws-kms.git?ref=v2.0.0"
```

to:

```
# renovate: version=v2.0.0
source = "git@github.com:terraform-aws-modules/terraform-aws-kms.git?ref=SOMESHA"
```

### Describe why we need/want these change(s).

Better reproducibility.

Blocked by #23248