feat(httpx): implement optional body logging also on http error (#651)

1. we want optionally to log the body (we don't want to log the body
when we're fetching psiphon secrets or tor targets)

2. we want body logging to _also_ happen on error since this is quite
useful to debug possible errors when accessing the API

This diff adds the above functionality, which were previously
described in https://github.com/ooni/probe/issues/1951.

This diff also adds comprehensive testing.

internal/engine/experiment/webconnectivity/control.go

@@ -62,7 +62,7 @@ func Control(
 	sess.Logger().Infof("control for %s...", creq.HTTPRequest)
 	// make sure error is wrapped
 	err = legacyerrorsx.SafeErrWrapperBuilder{
-		Error:     clnt.Build().PostJSON(ctx, "/", creq, &out),
+		Error:     clnt.WithBodyLogging().Build().PostJSON(ctx, "/", creq, &out),
 		Operation: netxlite.TopLevelOperation,
 	}.MaybeBuild()
 	sess.Logger().Infof("control for %s... %+v", creq.HTTPRequest, err)