fix(netxlite): improve TLS auto-configuration (#409)

Auto-configure every relevant TLS field as close as possible to
where it's actually used.

As a side effect, add support for mocking the creation of a TLS
connection, which should possibly be useful for uTLS?

Work that is part of https://github.com/ooni/probe/issues/1505

internal/engine/legacy/netx/dialer.go

@@ -108,7 +108,7 @@ func newTLSDialer(d dialer.Dialer, config *tls.Config) *netxlite.TLSDialer {
 		Dialer: d,
 		TLSHandshaker: tlsdialer.EmitterTLSHandshaker{
 			TLSHandshaker: tlsdialer.ErrorWrapperTLSHandshaker{
-				TLSHandshaker: &netxlite.TLSHandshakerStdlib{},
+				TLSHandshaker: &netxlite.TLSHandshakerConfigurable{},
 			},
 		},
 	}