Add ALPN, SNI, and negotiated protocol version support

[keatumal]

This PR:

• adds ALPN support
• adds methods for retrieving the negotiated protocol version
• adds SNI host name support
• includes small documentation updates

I used the following snippet for manual testing:

use OpenSSL;

my @hosts = <raku.org apple.com>;

for @hosts -> $host {
    say "--> $host";
    my $conn = IO::Socket::INET.new: :$host, :port(443);
    my $openssl = OpenSSL.new: :client;
    $openssl.set-fd: $conn.native-descriptor;
    $openssl.set-sni-host-name: $host;
    $openssl.set-alpn-protocols('h2', 'http/1.1');
    $openssl.connect;
    say '  version-code: ' ~ sprintf('0x%X', $openssl.version-code);
    say '  version-name: ' ~ $openssl.version-name;
    say '  protocol-version: ' ~ $openssl.protocol-version;
    say '  selected-alpn: ' ~ $openssl.selected-alpn;
    $conn.close;
}

On my Linux machine this prints:

--> raku.org
  version-code: 0x304
  version-name: TLSv1.3
  protocol-version: 1.3
  selected-alpn: h2
--> apple.com
  version-code: 0x304
  version-name: TLSv1.3
  protocol-version: 1.3
  selected-alpn: http/1.1

[keatumal]

I added online integration tests in xt/ and a small helper module for them.
The tests compare negotiated TLS version and ALPN with curl.

[timo]

LGTM