Add the ability to set a mirror URL to the project API endpoint
### There is no way to set a mirror URL for a project
Right now we have 5 different mirroring settings for a project in the API, but none of them seem to allow setting the remote URL to mirror.
### Target audience
- Devon, DevOps Engineer, https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas#devon-devops-engineer
### Further details
Right now it looks like the only way to set a remote URL to mirror is via the web UI. For anyone looking to mirror a lot of repositories, going through that process hundreds of times would be incredibly painful.
We can currently set:
mirror - on/off
mirror_user_id int
mirror_trigger_builds on/off
only_mirror_protected_branches on/off
mirror_overwrites_diverged_branches on/off
But there appears to be now way to set a remote "Git repository URL" like we can in the web UI.
### Proposal
Add an attribute to the projects API to allow for setting the remote URL.
### Permissions and Security
The security is already covered by the projects API
issue