Remove http client request body capturing 1024 byte limit

We would like to extend the HTTP client request body capturing feature to support more than 1024 bytes.
The 1024 bytes limit currently comes from the fact that we send the content as `otel.attribute` in Intakev2, which only supports 1024 bytes.

Therefore we'll have to extend the Intakev2 protocol with a separate field which doesn't have this limitation, like we already do for transactions.

- [ ] Add support for `http.request.body` intakev2 field to spans (copy it from transactions `request.body`)
- [ ] Send the http client requests bodies in `http.request.body` instead of `otel.attributes`
- [ ] Drop the 1024 bytes limitation when capturing the bodies