Add template function for returning the path to the pack's directory

[josegonzalez]

Description

When distributing a pack, it is common to wish to ship larger config files as part of a pack in separate files. This allows for better readability, as well as ensuring users can fallback on syntax highlighting from their editors when working on those files.

Reminders

• Add CHANGELOG.md entry

---

Usage:

      template {
        data        = <<EOF
[[ fileContents (list (packPath) "templates/configs" "file-name.txt" | join "/") ]]
EOF
        destination = "${NOMAD_TASK_DIR}/file-name.txt"
      }

[josegonzalez]

Is there anything I need to do to get this merged?