Invite non-members to project via email using API

Description

Currently, members can only be added to a group or project by specifying user_id (https://docs.gitlab.com/ce/api/members.html#add-a-member-to-a-group-or-project)

Proposal

Group owners should also be able to invite members who don't currently have an account using their email.

Links / references

https://docs.gitlab.com/ce/api/members.html#add-a-member-to-a-group-or-project

Documentation blurb

Nill