Add workflow for private github dependencies

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.

The issue names the Node buildpack and the referenced heroku-buildpack-github-netrc; start by reviewing those entry points and the proposed NODE_BUILDPACK_GITHUB_AUTH_TOKEN flow. Done means private GitHub dependencies can authenticate without tokens in Git history, and the workflow is documented.

Written by the indexing model from the issue text.