Switch from httr to httr2

[hadley]

And add rate limiting. Fixes #67.

[hadley]

@gaborcsardi if we decided to go down this path, I'll consider testing more broadly, doing some more refactoring and adding tests to get better coverage. But I have invested relatively little time at this point, so it would be easier enough to give it up.

A few reasons why it'd be worthwhile to switch to httr2:

• Adding retry with delay is a single line
• last_response() and last_request() make debugging in downstream packages much easier
• Could likely do further refactoring to use req_template() (but I think that'll require some httr2 changes because of the way that gh puts unmatched args in the body)

[hadley]

Hmmm, those tests assume that you have a PAT created with our default scopes. I can skip if the scopes aren't as expected, if you want.

[gaborcsardi]

Yeah, I think we need to skip, or add back the GH_TESTING token on the CI and skip if it is not set, because GITHUB_TOKEN might not have the same scopes for all the developers and also for the CI jobs.