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().

internal/engine/session.go

@@ -390,7 +390,7 @@ var ErrAlreadyUsingProxy = errors.New(
 // NewExperimentBuilder returns a new experiment builder
 // for the experiment with the given name, or an error if
 // there's no such experiment with the given name
-func (s *Session) NewExperimentBuilder(name string) (*ExperimentBuilder, error) {
+func (s *Session) NewExperimentBuilder(name string) (ExperimentBuilder, error) {
 	return newExperimentBuilder(s, name)
 }