Add new `urls` to metadata

One thing we thought about when listening to JD Long's keynote talk at posit::conf last month was the value of making sure it is as easy as possible connect datasets to documentation. This PR is inspired by those thoughts and adds a new option for storing urls (a character vector) along with your dataset.

It's used like this:

library(pins)

b <- board_connect()
#> Connecting to Posit Connect 2023.07.0 at <https://colorado.posit.co/rsc>
b |> pin_write(
    c(200, 404, 503),
    "http-code-numbers",
    urls = c("https://httbey.com/", "https://http.cat/"),
    force_identical_write = TRUE
)
#> Guessing `type = 'rds'`
#> Writing to pin 'julia.silge/http-code-numbers'

^{Created on 2023-10-18 with reprex v2.0.2}

I added the URLs to the Connect preview, which you can see here: https://colorado.posit.co/rsc/httpbey-pin/

Any thoughts on a better way to present that?

rstudio / pins-r

Add new `urls` to metadata #795