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)

[gaborcsardi]

I am fine with switching to httr2, it also helps httr2 to get more usage.
Looks good, I left some comments. In particular, we could make sure that returning list() for an empty response does not break anything in usethis.

Some tests cases are failing for me, I think these might be fragile.

Failure (test-gh_whoami.R:10): whoami works in presence of PAT
res\[\["scopes"\]\] does not match "\\buser\\b".
Actual value: "delete:packages, delete_repo, read:org, repo, workflow, write:packages"
Backtrace:
 1. testthat::expect_match(res[["scopes"]], "\\buser\\b")
      at test-gh_whoami.R:10:2
 2. testthat:::expect_match_(...)

Error (test-mock-repos.R:33): can POST, PATCH, and DELETE
<github_error/http_error_404/rlang_error/error/condition>
Error in `gh_process_response(raw)`: GitHub API error (404): Not Found
x URL not found: <https://api.github.com/gists>
i Read more at <https://docs.github.com/rest/reference/gists#create-a-gist>
Backtrace:
 1. gh::gh(...)
      at test-mock-repos.R:33:2
 2. gh:::gh_process_response(raw)
      at gh/R/gh.R:176:2

Error (test-mock-repos.R:33): can POST, PATCH, and DELETE
<github_error/http_error_404/rlang_error/error/condition>
Error in `gh_process_response(raw)`: GitHub API error (404): Not Found
x URL not found: <https://api.github.com/gists>
i Read more at <https://docs.github.com/rest/reference/gists#create-a-gist>
Backtrace:
 1. gh::gh(...)
      at test-mock-repos.R:33:2
 2. gh:::gh_process_response(raw)
      at gh/R/gh.R:176:2

[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.