Allow users to commit using both corporate and external email addresses

**Is your feature request related to a problem? Please describe.**
This is a variant of #1210. We have many open-source contributors who wish to commit using their external GitHub email addresses. We’d like to extend the git-proxy service to support this.

At present, each user has a single email address associated with their account — in our case, the corporate email provided via SSO.

When a user pushes a branch, git-proxy uses this email to verify whether the user is authorised to contribute to the target repository. Commits from any other email address are rejected.

**Describe the solution you'd like**
Allow users to add additional external email addresses.

Extend `checkUserPushPermission` to validate authorisation against any verified email address linked to the permitted contributor.