These methods mirror the existing methods that fetch the peer certificate and
chain. Due to various factors (SNI, multiple key types, etc.) it is not always
trivial for an application to determine what certificate was presented to the
client. However there are various use-cases where this is needed, such as
tls-server-end-point channel binding and OCSP stapling.
Requires OpenSSL 1.0.2+ (note: SSL_get_certificate() has existed for a very
long time, but was lacking documentation until OpenSSL 3.0).