Surveys partial responses requires sending duplicated data in the event

Bug Description

In our current implementation of partial responses, on each survey sent events, we're always sending duplicated data.

On the first event, we send:

$survey_response_<question-id-1>: 10

Second:

$survey_response_<question-id-1>: 10
$survey_response_<question-id-1>: 'Hello'

And so on.

Maybe there's a way for us to handle merging the events together on the server side. This would allow us to send events with less payload, and then we can join the responses together by the $survey_submission_id.

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables or https://eu.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]