Create issue with custom username as API

Description

The ability to create issues as unregistered/custom usernames. This could be handy for importing purpose, but also while connecting GitLab to your support system for example. On this way you're able to integrate GitLab issues even more with our CRM/support system, to have a central place to manage issues.

Proposal

API parameter on creating issue / posting as a reply on issues to define a custom username (with custom avatar, e-mail and/or clickable-URL).

Links / references

https://api.slack.com/methods/chat.postMessage (as_user and username)