Support secure interactions with private repositories 🔒

[JamieSlome]

GitProxy is able to generate server-side diffs for public repositories without having to authorise with the upstream repository. This is because the nature of open source is to allow the cloning of public repositories directly without assignment of permissions.

To support private repositories that require a layer of authorisation, to generate a server side diff on behalf of the developer we have to consider how to safely and securely connect to the upstream and protected codebase without compromising the security of the developer's credentials or personal access token.