Implement support for fetch project members

[mburtscher]

Right now it is only possible to fetch group members. However fetching project members would be as much needed.

https://docs.gitlab.com/ee/api/projects.html#project-members

[jetersen]

I'd like to refactor group members into a shared resource to not duplicate code.

Much like node-gitlab has it https://github.com/node-gitlab/node-gitlab/blob/master/src/core/templates/ResourceMembers.ts https://github.com/node-gitlab/node-gitlab/blob/master/src/core/services/ProjectMembers.ts https://github.com/node-gitlab/node-gitlab/blob/master/src/core/services/GroupMembers.ts