refactor(engine): *http.Client -> model.HTTPClient (#836)

This diff makes the implementation of the engine package more
abstract by changing HTTPClient() to return a model.HTTPClient
as opposed to returning an *http.Client.

Part of https://github.com/ooni/probe/issues/2184

internal/engine/session.go

@@ -337,7 +337,7 @@ func (s *Session) GetTestHelpersByName(name string) ([]model.OOAPIService, bool)
 }
 
 // DefaultHTTPClient returns the session's default HTTP client.
-func (s *Session) DefaultHTTPClient() *http.Client {
+func (s *Session) DefaultHTTPClient() model.HTTPClient {
 	return &http.Client{Transport: s.httpDefaultTransport}
 }