flags and plugins as lists (#2094)

# Description

Mixpanel is great at dealing with list values, especially in breakdowns.
This PR sends `alpha_flags` and `plugins` as lists instead of prefixes
key/values. Saves some network traffic too! Note: only sends enabled
flags, which I think is what we are interested in.



https://github.com/user-attachments/assets/62233148-2893-45ba-9dc0-152f6664b667



## Changelog

- [x] Patch
- [ ] Skip

## cdf

### Improved

- Optimized logging of flags and plugins

## templates

No changes.
