Allows to work with execute permissions in GitLab API

What does this MR do?

Adds chmod action to POST /projects/:id/repository/commits API.

With this action the user can update the execute_filemode of a given file in the repository.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #43832 (closed)