Stop setting `event.request` in Browser SDKs

[mydea]

Description

Today, we set data about the current page on event.request in the Browser SDK. This is done by the HttpContext integration:

 // if none of the information we want exists, don't bother
      if (!WINDOW.navigator && !WINDOW.location && !WINDOW.document) {
        return;
      }

      // grab as much info as exists and add it to the event
      const url = event.request?.url || getLocationHref();
      const { referrer } = WINDOW.document || {};
      const { userAgent } = WINDOW.navigator || {};

      const headers = {
        ...event.request?.headers,
        ...(referrer && { Referer: referrer }),
        ...(userAgent && { 'User-Agent': userAgent }),
      };
      const request = {
        ...event.request,
        ...(url && { url }),
        headers,
      };

      event.request = request;

In product, we show this as "HTTP Request Information" which is pretty confusing. We should stop setting this for browser SDKs, and instead set this information in a different place (e.g. in a page context, TBD).

We can then think about instead using this place to store outgoing-request information for failed fetch calls.

[lforst]

Awful things:

• Without the url in the request interface, the url tag in the event isn't prefilled. I have no idea what downstream stuff depends on this.
• Without the user agent as a header, relay doesn't know what browser to attribute to (especially when tunneling)

I feel like we need a spec for a Page context and let relay pick these things up from there.

[timfish]

There's already a browser context which I'm guessing would be most appropriate to move the user agent to.

[lforst]

Yup I already have a relay PR upen that is stuck in limbo to backfill the url in the browser context so we can slowly phase out event.request generically: getsentry/relay#4659