fix: resolve peer certificate symbols dynamically on Windows

[canermastan]

Summary

Fixes compilation with -d:ssl on Windows when using the asyncdispatch backend.

std/openssl does not declare SSL_get_peer_certificate or X509_free
on Windows, so directly calling them prevents dependent projects from
compiling.

This resolves the OpenSSL symbols dynamically instead:

• prefers SSL_get1_peer_certificate for OpenSSL 3
• falls back to SSL_get_peer_certificate for older OpenSSL versions
• resolves X509_free from libcrypto

This preserves SCRAM-SHA-256-PLUS channel binding when the installed OpenSSL
library supports it. If the required symbols are genuinely unavailable,
cbPrefer falls back safely and cbRequire continues to fail explicitly.

Related downstream report:
canermastan/jazzy-framework#33

Validation

Tested on Windows with Nim 2.2.12:

• nim check -d:ssl async_postgres.nim
• nim check tests/test_ssl.nim
• compiled a Jazzy application with -d:ssl against this branch