tls: add tlsSocket.socket, to access the raw stream beneath TLS

[pimterry]

If you created a TLS socket on top of an existing socket or duplex stream (new tls.TLSSocket(socket), tls.connect({ socket }), tlsServer.emit('connection', socket)) it's not possible to get the underlying socket back from the TLS socket through any public APIs.

Currently the only route to do this is the private _parent and _handle._parentWrap references, both of which are used in the ecosystem (including by my code) to workaround this issue. It would be nice if that wasn't necessary.

This PR adds a .socket property to TLS sockets, matching httpReq.socket and http2Session.socket. In reality in all 3 cases this might not actually be an actual socket - all 3 can run over any duplex - but the consistency is nice (it matches the option naming too) and it's clear enough imo.

There is notably one common case where this is still null: TLS client connections, made directly like tls.connect{ host, port }) (not using an existing socket). In this case there's no underlying net.Socket or stream object created, so there's nothing to expose. I think that's OK: the TLSSocket itself here is the raw socket, so there's no parent to reach and null is correct. It's noted in the docs here.

End result: when layering protocols on top of each other (e.g. H2 over TLS over H1 CONNECT), with this change you can just loop up the .socket references and you'll always (I think) eventually reach the real socket instance.

[nodejs-github-bot]

Review requested:

• @nodejs/crypto
• @nodejs/net

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.44%. Comparing base (6d5e309) to head (20f2a00).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66598      +/-   ##
==========================================
- Coverage   90.44%   90.44%   -0.01%     
==========================================
  Files         791      791              
  Lines      276562   276565       +3     
  Branches    53126    53119       -7     
==========================================
- Hits       250130   250127       -3     
+ Misses      16846    16844       -2     
- Partials     9586     9594       +8     

Files with missing lines	Coverage Δ
lib/internal/tls/wrap.js	95.62% <100.00%> (+<0.01%)	⬆️

... and 28 files with indirect coverage changes

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