fix(plugin-server): assert fields we iterate over are objects

[bretthoerner]

This is a WIP that will require tests to be updated if we actually want to merge it.

However, it [seems to me that capture-rs will reject events] if the user sends a string as the toplevel properties, $set, or $set_once. We could just add an assertion for properties.$set and properties.$set_once and reject those events too, which seems cleaner...

Anyone have opinions on whether we should just reject these events as misshapen in capture?

Worth noting that Python capture definitely doesn't require (or check) that these are objects, to our detriment: https://posthog.sentry.io/issues/5321764413/

[posthog-bot]

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[posthog-bot]

This PR was closed due to lack of activity. Feel free to reopen if it's still relevant.