Support executable flag in repository files API

Merged Timo Furrer requested to merge tuxtimo/gitlab:feature/repository-file-api-exec-mode into master

This change adds support for a execute_filemode field in the Repository Files API endpoints, much like it's already available in the Commits API.

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.

/cc @nagyv-gitlab @nmezzopera

Edited by Timo Furrer