feature: support client_hello_by_lua with boringssl

[swananan]

I hereby granted the copyright of the changes in this pull request to the authors of this lua-nginx-module project.

The Lua-nginx-module recently added support for building with BoringSSL. However, due to differences between the APIs of OpenSSL and BoringSSL, the client_hello_by_lua directive wasn't supported. Therefore, I submitted this pull request. There will be other pull requests for stream-lua-nginx-module and lua-resty-core.