Use GitHub App for context auth instead of PAT

**What is the problem you're trying to solve**
When using a private org for GitHub you can block PAT tokens outside the network and allow Apps only, or you could just prefer the better security for GitHub Apps.

Currently it seems the builder only support PAT for private repo auth.

**Describe the solution you'd like**
An alternative to --git-access-token that allows the use of a GitHub App for auth to download source and/or hooks up to events.

**Additional context**
https://docs.github.com/en/apps

Start by tracing how the --git-access-token entry point is handled for private repository access. Review the linked GitHub Apps documentation and define the authentication flow for downloading source and subscribing to hooks; done means a GitHub App alternative works alongside PAT authentication.

Written by the indexing model from the issue text.