Add `page` context and infer data from it (WIP)

[mydea]

Based on getsentry/sentry-docs#13203, let's wait for this to be merged so details are settled.

We want to add a new page context, based on the develop docs schema:

{
  "contexts": {
    "page": {
      "url": "https://sentry.io/issues",
      "http.query": "?page=1",
      "http.fragment": "#123",
      "referer": "https://sentry.io/"
    }
  }
}

The only required field is url, all others are optional.

When this is present, we want to infer the url from it instead of from event.request.url.
I am not sure if we do anything with referer right now, but if we do, we should also infer it from there instead.