Add a Playwright fixture that allows retrieving POST request parameters

[obulat]

Problem

Currently, we use the following code to extract the Analytics payloads from POST requests in Playwright tests:

 page.on("request", (req) => {
    if (req.method() === "POST") {
      const requestData = req.postDataJSON()
      if (requestData?.n == "SELECT_EXTERNAL_SOURCE") {
        eventData = JSON.parse(requestData?.p)
      }
    }
  })

This code is repeated in every test that tests analytics.

Description

Abstract this out into a utility or custom Playwright fixture that provides this more generally (like a list of all events that get sent that can be asserted against).

Alternatives

Additional context

Based on quote from @sarayourfriend 's comment

[zackkrida]

Added in #1158