feat(tracking): add experiment category to tracking types

Adds an `experiment` category and `contentful_experiment_assignment_event` to EventDataTypes. This will help us to send Segment tracking events for Optimizely experiments from portal-app to the monolith.

This PR is related to:
Add new Segment Tracker event to monolith: codecademy-engineering/Codecademy#30278
Adding tracking event trigger on an decision event: codecademy-engineering/portal-app#1787

We're trying to get a tracking event fired like this:
![Screen Shot 2022-04-06 at 12 06 41 PM](https://user-images.githubusercontent.com/29311886/162050123-5fc0fbe1-8301-4a8f-880e-871f1e47d9b1.png)

packages/tracking/src/events/types.ts

@@ -42,6 +42,10 @@ export type EventDataTypes = {
   exercise: {
     force_pass: BaseEventAnyData;
   };
+  // events for tracking experiments in optimizely
+  experiment: {
+    contentful_experiment_assignment_event: BaseEventAnyData;
+  };
   payments: {
     cancel_survey: BaseEventAnyData;
   };