feat(netxlite): use *Netx for creating DoH resolvers (#1255)

While working on removing technical debt, as part of
ooni/probe#2531, it makes sense ensuring that
there is a single way of constructing netxlite types by always using an
instance of *Netx. This job is relatively easy because we are a couple
of patches away from achieving this goal and, by doing this, we would
avoid creating future technical debt. So, I am going to ahead and
implement this.

Also, apparently, the oohelperd uses this functionality and
somehow we missed doing this when converting it to using netemx. We
should fix this soon. (This issue immediately validates what I was
saying above about techdebt 😅)

internal/netxlite/resolvercore.go

@@ -39,12 +39,19 @@ func NewStdlibResolver(logger model.DebugLogger) model.Resolver {
 // NewParallelDNSOverHTTPSResolver creates a new DNS over HTTPS resolver
 // that uses the standard library for all operations. This function constructs
 // all the building blocks and calls WrapResolver on the returned resolver.
-func NewParallelDNSOverHTTPSResolver(logger model.DebugLogger, URL string) model.Resolver {
-	client := &http.Client{Transport: NewHTTPTransportStdlib(logger)}
+func (netx *Netx) NewParallelDNSOverHTTPSResolver(logger model.DebugLogger, URL string) model.Resolver {
+	client := &http.Client{Transport: netx.NewHTTPTransportStdlib(logger)}
 	txp := wrapDNSTransport(NewUnwrappedDNSOverHTTPSTransport(client, URL))
 	return WrapResolver(logger, NewUnwrappedParallelResolver(txp))
 }
 
+// NewParallelDNSOverHTTPSResolver is equivalent to creating an empty [*Netx]
+// and calling its NewParallelDNSOverHTTPSResolver method.
+func NewParallelDNSOverHTTPSResolver(logger model.DebugLogger, URL string) model.Resolver {
+	netx := &Netx{Underlying: nil}
+	return netx.NewParallelDNSOverHTTPSResolver(logger, URL)
+}
+
 func (netx *Netx) newUnwrappedStdlibResolver() model.Resolver {
 	return &resolverSystem{
 		t: wrapDNSTransport(netx.newDNSOverGetaddrinfoTransport()),