Add workflow for private github dependencies

[jmorrell]

Using private github repos as dependencies is a common pattern, but people are often tripped up with the auth flow. It seems a lot of people end up here: https://stackoverflow.com/questions/10869796/npm-private-git-module-on-heroku/13353408#13353408 but the top answer involves putting access tokens into your git history, which we should not encourage.

Instead, we should build https://github.com/timshadel/heroku-buildpack-github-netrc into the Node buildpack and document its use. We can use a special env var like NODE_BUILDPACK_GITHUB_AUTH_TOKEN and generate the .netrc for the user.

[josegonzalez]

Potentially a duplicate of #174?

[JestDotty]

174 allows you to use private npm repositories

there's a need for git repos with private projects. Would need a way to set github/gitlab SSH configs