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/experiment_test.go

@@ -14,7 +14,7 @@ func TestExperimentHonoursSharingDefaults(t *testing.T) {
 		if err != nil {
 			t.Fatal(err)
 		}
-		exp := builder.NewExperiment()
+		exp := builder.NewExperiment().(*experiment)
 		return exp.newMeasurement("")
 	}
 	type spec struct {