Include (3) email types in User API

The following discussion from !56164 (merged) should be addressed:

• @mjang1 started a discussion: (+4 comments)

Based on the discussion, we've determined that we have 3 types of emails for each user:

• Primary email
• Public email
• Commit email

We should see all 3 in the Users API. Currently we do not see the "Commit email" address. So, we should:

• Add commit email to the Users API
• Update the Users API doc accordingly, ref https://docs.gitlab.com/ee/api/users.html