ssl_client_hello_by_lua lacks get_client_hello_ext_present

[climagabriel]

Hi,

Not to sound entitled but it's a big omission that you can get_client_hello_ext(42) to get the 42nd extension but can't check what extensions are even present, before you do it. The only way to do this currently is to iterate on get_client_hello_ext() and hope that you didn't miss something.

OPENSSL does provide a clean way to this though:
SSL_client_hello_get1_extensions_present().

I'm working to implement this myself. But if anyone wants to pitch in bless you, thx

[climagabriel]

thanks @zhuizhuhaomeng for helping me get these changes merged