Request: re-export `curl::has_internet()` or add equivalent internet check function

[m-muecke]

For testing and examples it would be great if httr2 had a function for checking internet connection, like testthat::skip_if_offline() or curl::has_internet(). Would it be perhaps possible of re-exporting the curl::has_internet() function?

[hadley]

My biggest hangup here is what to call the function — currently all httr2 functions either start with req_, resp_ or url_, but none of those feel quite right. Maybe it's ok to introduce a new prefix for just this function, but that also doesn't feel quite right.

[hadley]

Hmmm, if we implement #435 this could be host_online().

[hadley]

Hmmm, looking at curl::has_internet() more closely I'm pretty sure that we want to use its behaviour, rather than testthat:::has_internet(). But maybe it's ok to have httr2::is_online()