Add support for reading authenticated container repositories for Kptfile functions

[ChristopherFry]

Add support to allow Porch to access authenticated container repositories for Kptfile function images. Currently Porch can only pull images from public repositories where no authentication is required, or from gcr.io using workload identity.

One specific use-case, requested by the community, is to allow private function images to be pulled from ghcr.io using a GitHub Personal Access Token.