add lua_ssl_certificate for server to certificate client

[rainingmaster]

I hereby granted the copyright of the changes in this pull request
to the authors of this lua-nginx-module project.
I have add lua_ssl_certificate ported from proxy_module, and try to solve problem like here. When use ngx.tcp.socket, we can send our certificate to server to pass verification. we have use like this in our private module, is it suitable?
If it is suitable, I will add more test cases~

[agentzh]

@rainingmaster Maybe we should make it explicit that we are supporting client certificates here? The current names are a bit confusing.

I think maybe it's better to set this in the Lua API instead since this global setting affects all the SSL cosockets, which may give surprises. Like in this PR: #997

[rainingmaster]

OH, I kwnow. Yes, that is more flexible in that way.

[CodingMinds]

@rainingmaster Is there a new PR ?

[agentzh]

@CodingMinds Yes. See #997.