Add support for server-sent events

[hadley]

https://developer.mozilla.org/en-US/docs/Web/API/Server-sent%5Fevents/Using%5Fserver-sent%5Fevents#event_stream_format

Maybe req_perform_sse()? And then have some way to register functions for each event?

cutpoint <- function(bytes) {
  nl <- which(bytes == charToRaw("\n"))
  nl[nl %in% (setdiff(nl, 1) - 1)]
}

parser <- function(bytes) {
  text <- rawToChar(bytes)
  events <- strsplit(text, "\n\n")[[1]]
  lines <- strsplit(text, "\n")
  ...
}

req |> 
  req_perform_stream(round = cutpoint, callback = parser)

[hadley]

Done in #521