Support multiple values for `type`, at least for reading

[juliasilge]

In rstudio/pins-r#877, we are adding support for reading and writing multiple values for type, as outlined in rstudio/pins-r#788.

In R, it will look like:

board |>
  pin_write(account_information, 
            name = "account_information", 
            type = c("parquet", "csv"))

You can see an example pin that has multiple types at https://pub.demo.posit.team/content/d97baef7-228e-4eee-8e45-79bd8aaeca71; let me know if you have trouble accessing that.

I think the minimum support we'd want on the Python side is being able to read one of these, something like this with a new optional type:

board.pin_read("hadley/sales-summary", type = "parquet")

Full support would look like reading and writing.