Pull images using PAT for the Github repository

[4molybdenum2]

Currently Github authentication requires us to create a Personal Access Token (PAT). This same token can be used to pull images from ghcr. Another alternative is configuring a default secret at the service account level: https://kubernetes.io/docs/tasks/configure-pod-container/configure-service-account/#add-imagepullsecrets-to-a-service-account which will help us in fetching the image on a private registry

PS: Is it possible to remove the requirement for PAT altogether and use something like Github Authentication Provider for Backstage? https://backstage.io/docs/auth/github/provider#docsNav

[ChristopherFry]

Created kptdev/kpt#3434 to add support to Porch for authenticated container repositories. Once this is complete, I'll be able to add support to the UI.