This change adds support for a `execute_filemode` field in the
[Repository Files
API](https://docs.gitlab.com/ee/api/repository_files.html)
endpoints, much like it's already available in the
[Commits API](https://docs.gitlab.com/ee/api/commits.html).

The driver for this change is the `gitlab_repository_file` resource in
the Terraform GitLab Provider. This resource allows to manage a file
using the Repository Files API, but wasn't supporting to control the
executable flag on those files.

Changelog: added
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/83499