Refactor (or replicate) common CLI packages into separate module for binary extension developers

[heaths]

Describe the feature or problem you’d like to solve

Script extensions aren't always ideal, especially when extensions may want to do more that is difficult or otherwise very involved with scripts. Writing binary extensions in Go would be useful in these cases, but then devs have to mimic or try to go get the CLI which isn't really a target goal of the executable-producting projects.

Proposed solution

In support of binary extensions, it would be great if a lot of the functionality like IOStreams, TablePrinter, authentication, command factories, etc. - even testing facilities where possible - were refactored out or otherwise replicated into a go-gettabke module. This would provide the same great packages you use in the CLI to extension developers to hopefully present a consistent UX for end users.

[samcoe]

@heaths I am currently getting started on this 👍 I will update here when I have made more progress.

[samcoe]

We just released the beta version of go-gh 🥳, please check it out and leave feedback in the repo!

[samcoe]

Reopening per discussion: cli/go-gh#5 (comment).