Support for p12 certificate format

in curl it is possible to use .pfx aka PKCS12 as a single file for client certificate authentication
i.e " curl --cert certificate.pfx --cert-type p12 ..."

would love to see the support for it in grpcurl as well to avoid breaking up the .pfx into 2 files of cert / key and in cases where the passcode is baked in and unknown by the user

Start at grpcurl’s existing client-certificate authentication options and TLS setup, then compare their behavior with curl’s .pfx/PKCS12 support. Identify how a .pfx file and its password are currently expected to enter the command, and define coverage for using the bundle directly. Done means client authentication accepts the requested certificate format without requiring separate certificate and key files.

Written by the indexing model from the issue text.