Adding Endpoint to add team members directly to project by admin user

[RishiRajSahu]

With the incorporation of team invites we don't have ability of directly adding a member to team via rest endpoint directly by Admin. Currently the member addition endpoint is only supporting addition of the requesting user, we won't be able to add members to the project via API which might cause some troubles to support team. So, we should create a new endpoint to provide that functionality back.

for more details - #212 (comment)
fyi - @vikasrohit