Add ParameterDescription message to libpq frontend long message types

Hi,

I'm maintaining a Rust library pgwire to implement postgres wire
protocol in rust. While doing a corner case test, I noticed the
inconsistency for ParameterDescription between backend and frontend.

The backend allows up to 65535 parameters in a prepared statement. But
when running Describe on the statement, there is a size limit of 30000
bytes for ParameterDescription on the frontend. This means we can only
describe statements with at most ~7500 parameters. For statements exceed
the limit, it ends up with error about the message size.

This patch simply adds ParameterDescription to VALID_LONG_MESSAGE_TYPE
whitelist to remove the cap.