hclfmt: accept multiple files

[Mic92]

Description

hclfmt: accept multiple files when using the --terragrunt-hclfmt-file option. This is useful when used in combination with https://github.com/numtide/treefmt

TODOs

Read the Gruntwork contribution guidelines.

• [x] Update the docs.
• [x] Run the relevant tests successfully, including pre-commit checks.
• [x] Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• [x] Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

• The --terragrunt-hclfmt-file option in hclfmt now supports multiple files.

Migration Guide

[denis256]

Tests fail to run since cli/hclfmt_test.go is not compilable, it has reference to tgOptions.HclFile