`gh pr checkout` should add the fork as a remote

# CLI Feedback

## What have you loved?

`gh pr checkout` is one of the best features of this tool, thanks!

## What was confusing or gave you pause?

I had a PR I created elsewhere to an org repo, requesting changes from my own fork's branch, e.g.:

`upstream` has PR `#2` with changes from `origin:patch-1` branch to `upstream:main`.

In a new computer I clone the repo with `gh clone org/project`, then on the project's folder, I checkout the PR's work with `gh pr checkout 2`. It gets me on the `patch-1` branch, but my `git remote` still points only the the org's remote, not to my fork's remote.

__My issue__ happened when I applied changes to the branch and tried to push it back. It's a minor problem but I can tell that using `git remote add leobalter git@github.com:leobalter/.git` is something that I'd love to avoid when I use `gh pr checkout`.

## Are there features you'd like to see added?

I'd love if `gh pr checkout` could add the remote by default if it's not listed in the known remotes, or at least offer me an option to add it. This would allow a quick way to set the new branch to track the proper remote branch as well.