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/probeservices/bouncer.go

@@ -9,6 +9,6 @@ import (
 // GetTestHelpers is like GetCollectors but for test helpers.
 func (c Client) GetTestHelpers(
 	ctx context.Context) (output map[string][]model.OOAPIService, err error) {
-	err = c.APIClientTemplate.Build().GetJSON(ctx, "/api/v1/test-helpers", &output)
+	err = c.APIClientTemplate.WithBodyLogging().Build().GetJSON(ctx, "/api/v1/test-helpers", &output)
 	return
 }