Delegate requests to httr2

[dfalbel]

This is a draft PR for discussion. The idea here is to centralize all requests made by ragnar to use httr2 and add a callback allowing users to modify requests. This allows users to use a central cache that can be shared for both finding links and reading documents. It also allows users to modify requests such that they can be retried, have a timeout, etc.

Here's an example:

modify_request <- function(req) {
  req |>
    httr2::req_cache(path = "cache")
}


links <- ragnar::ragnar_find_links(
  "https://quarto.org",
  depth = 3,
  children_only = FALSE,
  url_filter = function(x) {
    # We only want quarto links from official repositories
    # - quarto.org
    stringi::stri_subset(x, regex = "^https://quarto.org")
  },
  modify_request = modify_request
)

Then links can be read with:

doc <- link |>
      ragnar::ragnar_read(
        frame_by_tags = "h1",
        modify_request = modify_request
      )

Another idea, would be for find_links to return httr2_response objects (eg being something very similar to what ragnar_perform_spider() would do r-lib/httr2#456. And ragnar_read being able to read httr2_response objects additionally.