refactor(engine): more abstract Experiment{,Builder} (#838)

This diff modifies the engine package to make Experiment and
ExperimentBuilder interfaces rather than structs.

The previosuly existing structs are now named experiment{,Builder}.

This diff helps https://github.com/ooni/probe/issues/2184
because it allows us to write unit tests more easily.

There should be no functional change.

While there, I removed a bunch of deprecated functions, which were
unnecessarily complicate the implementation and could be easily
replaced by passing them a context.Context or context.Background().

pkg/oonimkall/experiment.go

@@ -67,7 +67,7 @@ type experimentBuilder interface {
 
 // experimentBuilderWrapper wraps *ExperimentBuilder
 type experimentBuilderWrapper struct {
-	eb *engine.ExperimentBuilder
+	eb engine.ExperimentBuilder
 }
 
 // newExperiment implements experimentBuilder.newExperiment