Add support for events

[lambtron]

I was writing the GA blog post and wanted to include an example of sending an event to GA, then noticed that the message payload does not include any event params.

Supporting GA events would enable wider use of GA reporting (such as creating goals, ecommerce support, etc.) than simply page views. The downside is that it's not super clear how to enable users to add event data. Currently the ga function accepts a Request object.

Finally, i'm not sure how we wanna prioritize this if we're gonna add GA4 support.