feat: add tracks assignment and exposure

[zhukaihan]

Summary

Adds option to track assignment and exposure events, or not, to fetch requests.

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?: no

[zhukaihan]

Requires a new experiment-core version to update the version and type.

[kyeh-amp]

I'm still getting my bearings on this, so I apologize if is a trivial question: Is the intended use for this meant for to configure tracking on a per fetch request basis or for all requests for a customer?

If this is intended to be an option for individual fetch requests to be made with and without tracking, then this seems fine. However, if the feature is meant more for a means for a user to wholesale turn on/off tracking, then it might be a better idea to set this on a client config level to prevent having to remember to set the flags for each place you might call fetch in your codebase.