Make `is_interactive()` react to "rlang_interactive" option

[jennybc]

This will allow me to make gargle's pseudo-OOB flow work in Google Colab, which would be really nice.

Relates to https://github.com/r-lib/gargle/pull/237, where I currently have utils::assignInNamespace("is_interactive", rlang::is_interactive, ns = "httr") in .onLoad(), but I can remove that if this PR gets merged and released. I don't think I could release gargle to CRAN with assignInNamespace().

Relates to https://github.com/r-lib/gargle/issues/140

[jennybc]

Just to record this analysis somewhere: I have not replaced every call to base::interactive() with is_interactive(). Why not? Where else is base::interactive() called?

• BROWSE(): For my motivating case, which is Google Colab, utils::browseURL() does not work and we do indeed want to go down the non-interactive() branch.
• should_cache() (internal helper): gargle never hits this code, because caching is done in-house. But FWIW utils::menu() really does require a fully interactive session.
• Default args in init_oauth1.0(), init_oauth2.0(), oauth_listener(): these args are deprecated, so it doesn't matter.