Add new var: app_config_entries

[brk3]

This allows use of the new related functionality in
terraform-azurerm-caf, which allows for adding entries to an azure
appconfig instance in a more flexible way.

See, aztfmod/terraform-azurerm-caf#1221

PR Checklist

---

• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My code follows the code style of this project.
• I ran lint checks locally prior to submission.
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Description

Does this introduce a breaking change

• YES
• NO

Testing

[arnaudlh]

LGTM