Add SocketInfo and alpn/sni negotiation

[mmastrac]

Adds alpn and sni members to the SocketOptions dictionary to address part of #14 and #15.

Adds an info attribute to Socket to retrieve the negotiated alpn protocol.

I left the negotiated SNI out of the info attribute as it may be better to offer the full server certificate in that case and I don't see a concrete use case for it (the TLS connection will abort if SNI was ignored or incorrect).

Note that sni and alpn are valid for starttls connections as well so the specification allows them for either on or starttls connections. An example of Server Name Indication with STARTTLS can be seen here: http://www.postfix.org/TLS_README.html

Once #12 is landed, it may make sense to update the text to specify that ready resolves when a TLS socket has been fully negotiated and

[jasnell]

A key question for the startTls case... should alpn and sni be provided when the Socket is opened or when startTls() is called? I can see arguments for either.

[mmastrac]

A key question for the startTls case... should alpn and sni be provided when the Socket is opened or when startTls() is called? I can see arguments for either.

That's an interesting question and I might lean towards the latter. I am not aware of any protocols that may send SNI-influencing information in plaintext, but it feels more correct.

[dom96]

A key question for the startTls case... should alpn and sni be provided when the Socket is opened or when startTls() is called? I can see arguments for either.

I actually think it may make sense to have the new opened promise (introduced in #12) resolve to a SocketInfo. For the startTls case this will work nicely since startTls returns a new Socket.

[jasnell]

So instead of having socket.info it would be const info = await socket.opened ? That would work for me, I think.

[dom96]

Yeah, precisely.

[jasnell]

PR adding SocketInfo and clarifying a bit more about the opened promise. #19 ... once that lands, this PR can be updated to add the new alpn property to the SocketInfo dictionary.

[dom96]

Gentle ping to update this to use the new SocketInfo dictionary :)

[mmastrac]

@dom96 @jasnell Ah thanks.. let me try to get to this one today.

[mmastrac]

Updated w/alpn and sni fields. I added the SocketInfo attribute for now on the Socket but we can definitely move it around.