GHE Authentication for `library.get()`

**Describe the problem/challenge you have**
I'm new to the YTT ecosystem and have been evaluating it's use in a GHE environment. I haven't seen anything about this anywhere else, hence my posting here.

Our use case would be to have a library of templates that exists within a GHE repo that a service can reference anywhere within the broader org by loading the template like `library.get(github.com/carvel-dev/ytt-library-for-kubernetes/app)`. I've noticed that all of the libraries that are fetched via http are public github links. To better fit our use-case, it would be invaluable to have the ability to fetch templates from GHE.

If I'm mistaken and this functionality _is_ currently available, let me know and I'll close out my request.

**Describe the solution you'd like**
The ability to provide a GHE AUTH token to `library.get()` in order to fetch templates from repos that require AUTH to access.

---
Vote on this request

This is an invitation to the community to vote on issues, to help us prioritize our backlog. Use the "smiley face" up to the right of this comment to vote.

👍 "I would like to see this addressed as soon as possible"
👎 "There are other more important things to focus on right now"

We are also happy to receive and review Pull Requests if you want to help working on this issue.

The issue names library.get() but no source files, tests, or entry points. Start by locating library.get() and the HTTP fetching path, then determine how authenticated GitHub Enterprise access is configured and tested. Done means private GHE libraries can be fetched with an auth token.

Written by the indexing model from the issue text.