Use go-github to interact with GitHub from Magician

We currently directly build REST API requests for GitHub in the Magician - for example, https://github.com/GoogleCloudPlatform/magic-modules/blob/main/.ci/magician/github/get.go

We should use https://github.com/google/go-github instead, to align with other tooling we maintain. This may also allow us to not maintain our own API wrapper altogether.