Insertion log: support private repositories

Some components inserted into VS use private GitHub repositories, in particular Live Unit Testing. We can't read the commits without an API key. It feels like we should be able to add a parameter for this and just use it without too much trouble. Or maybe we could get it out of the key vault we use when running in AzDO.

The issue names no files, tests, or entry points. Start by locating the insertion-log code that reads commits from private GitHub repositories and determine whether credentials should be passed as a parameter or retrieved from the AzDO key vault. Done means Live Unit Testing components from private repositories can be processed through a supported credential path.

Written by the indexing model from the issue text.