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/netx/tlsdialer/tls_test.go

@@ -16,7 +16,7 @@ import (
 )
 
 func TestSystemTLSHandshakerEOFError(t *testing.T) {
-	h := &netxlite.TLSHandshakerStdlib{}
+	h := &netxlite.TLSHandshakerConfigurable{}
 	conn, _, err := h.Handshake(context.Background(), tlsdialer.EOFConn{}, &tls.Config{
 		ServerName: "x.org",
 	})