feat(server): Send feature flags on event if sendFeatureFlags is set

[dustinbyrne]

🚨 This branch is a work in progress

💡 Motivation and Context

The sendFeatureFlags configuration option was not being respected in the server-side SDK. This change makes it functional.

💚 How did you test it?

WIP, tests to come

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

Instructions and example for changelog

Please add an entry to the appropriate changelog:

• posthog/CHANGELOG.md (core module)
• posthog-android/CHANGELOG.md (android module)
• posthog-server/CHANGELOG.md (server module)

Make sure the entry includes this PR's number.
Example:

## Next
- Send feature flags on event if `sendFeatureFlags` is set ([#302](https://github.com/PostHog/posthog-android/pull/302))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description.

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