Allow specifying committers username/email via API

Summary

Per discussion in https://gitlab.com/gitlab-org/gitlab-ce/issues/20147, we'd like to implement the ability to specify a username/email when making commits thru the API.

~"feature proposal" api

Possible Solution (needs work)

• Update the create, update, and delete requests in the Files API to accept optional params to specify the committers username/email
• How can we make those params usable in file services so it results in properly setting the commiter details in the repository model?
  • Note: we'd like to use our admin user's API token and specify these options to commit as different Gitlab users

Links

• Files API
• Files Services
• Repository Model
• Repository Files Doc