Bitbucket support, or general git+sshkey

## Problem / Motivation

We use BBC, not GitHub

## Proposed Solution

It shouldn't matter what platform I use, ideally Scion doesn't need to have code to handle every possible git host, though the major hosts like BBC / GitLab would be nice additions

## Alternatives Considered

Is there some way to have arbitrary git repos with auth that is not tied to a particular host? Like using ssh keys?

Start by tracing how Scion currently handles Git hosts and repository authentication. Compare the requested Bitbucket/GitLab support with the alternative of arbitrary repositories authenticated by SSH keys; done should be defined as supporting the chosen approach without host-specific handling for every provider.

Written by the indexing model from the issue text.