Allow for templatefile recursion (up to 1024 depth default)

[cam72cam]

This changes templatefile to track call depth when building templatefile functions. It works fine, even when nested inside other function calls.

As mentioned in #402, due to the lack of tail recursion it can be memory intensive during evaluation. In my opinion this is a reasonable trade off and the default max depth should prevent most OOM crashes during reasonable use.

Example:

main.tf 
locals {
        x = templatefile("rec.tmpl", {})
}

rec.tmpl 
${ base64encode(templatefile("other.tmpl", {})) }

other.tmpl 
${ templatefile("rec.tmpl", {}) }

With TF_LOG=debug

TODO:

• Documentation changes to templatefile
• Changelog entry

Resolves #402

Target Release

1.7.0

[github-actions]

Reminder for the PR assignee: If this is a user-visible change, please update the changelog as part of the PR.