libpq: Add support for dumping SSL key material to file

libpq: Add support for dumping SSL key material to file

This adds a new connection parameter which instructs libpq to
write out keymaterial clientside into a file in order to make
connection debugging with Wireshark and similar tools possible.
The file format used is the standardized NSS format.

Author: Abhishek Chanda <[email protected]>
Co-authored-by: Daniel Gustafsson <[email protected]>
Reviewed-by: Jacob Champion <[email protected]>
Discussion: https://postgr.es/m/CAKiP-K85C8uQbzXKWf5wHQPkuygGUGcufke713iHmYWOe9q2dA@mail.gmail.com