Add support for token attr in project hooks API

What does this MR do?

This MR adds support for the token attribute in the project hook API.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Feature requested on a dependant project: https://github.com/gpocentek/python-gitlab/issues/170

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?