Data source for ACL policy document

[OJFord]

Is your feature request related to a problem? Please describe.
It's convenient to be able to define documents such as policy files in HCL, for validation, syntax highlighting, etc.

Presently the tailscale_acl resource (which at time of writing refers to the whole policy file) requires specifying the file in inline (hu)json.

Describe the solution you'd like
Analagous to aws_iam_policy_document, I'd like to be able to do:

data "tailscale_policy_document" "this" {
    grant {
        src = ["*"]
        dst = ["*"]
        ip  = ["*"]
    }

    # ...
}

resource "tailscale_policy_file" "as_hcl" { # nee tailscale_acl
  acl = data.tailscale_policy_document.this.json
}

Additional context
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/data-sources/iam_policy_document

I could probably contribute a PR for this if you're open to the idea.