Missing some REST API proxies

[niconoe-]

👋

Thank you for this awesome project! I'm currently using it to automatize creation and settings of GitLab projects, and I noticed that some REST API endpoints were not avaiable on your library.

From what I found, I discovered missing :

• API to update a deploy key on a project, required to change a deploy key name or grant access to push: https://docs.gitlab.com/api/deploy_keys/#update-deploy-key
• APIs to handle the push rule on a project: https://docs.gitlab.com/api/project_push_rules/

I already provided PRs #838 and #839 (hoping I did them well and respected the guidelines).

Please do not hesitate to let me know if you need me to enhance the tests, the documentation or anything else related to what I'm proposing.

Have a nice day!