Can't get peercert subject name in ppv2

[HJianBo]

The https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt has not supported the cert Subject Name, but we supported it with Common Name 😢

esockd/src/esockd_peercert.erl

Lines 26 to 32 in 26fffb6

	subject(nossl) -> undefined;
	subject(undefined) -> undefined;
	subject(Cert) when is_binary(Cert) ->
	esockd_ssl:peer_cert_subject(Cert);
	subject(PP2Info) when is_list(PP2Info) ->
	%%Notice: DN is not available in ppv2 additional info
	proplists:get_value(pp2_ssl_cn, PP2Info).

This issue was found while investigating emqx/emqx#9147