Exposure event settings for Trend experiments should be more prominent in creation flow

[jurajmajerik]

Feature request

Is your feature request related to a problem?

Right now when you set up a trend experiment you can only modify the exposure event after the experiment is set up in draft mode. By default it will use $feature_flag_call events which for us is a non-starter because of our SSR implementation. Once again this makes the likelihood of a non-posthog power user configuring this incorrectly.

Describe the solution you'd like

Describe alternatives you've considered

Additional context

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.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]