bpo-36076: Add SNI support to ssl.get_server_certificate. (GH-16820)

Many servers in the cloud environment require SNI to be used during the
SSL/TLS handshake, therefore it is not possible to fetch their certificates
using the ssl.get_server_certificate interface.

This change adds an additional optional hostname argument that can be used to
set the SNI. Note that it is intentionally a separate argument instead of
using the host part of the addr tuple, because one might want to explicitly
fetch the default certificate or fetch a certificate from a specific IP
address with the specified SNI hostname. A separate argument also works better
for backwards compatibility.

Automerge-Triggered-By: GH:tiran