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

In https://github.com/rstudio/pins-r/pull/877, we are adding support for reading and writing multiple values for type, as outlined in https://github.com/rstudio/pins-r/issues/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.

Start at the board.pin_read entry point and compare it with the example pin linked in the issue, then trace how its type value is consumed. Done means the agreed Python-side scope—at minimum reading a selected type, or full read/write support if chosen—works with a pin containing multiple types.