Do not capture $set event if user props have not changed

[marandaneto]

Description

Context PostHog/posthog-android#251

Description

https://github.com/PostHog/posthog-js/blob/c8c747a53c4768686542899631ebbe440ab39760/src/posthog-core.ts#L1589-L1593

Relates to PostHog/posthog-android#194

• Android/Flutter
• iOS/Flutter
• RN - requires this too

https://github.com/PostHog/posthog-android/blob/b999daa0ea32d6a21a215ad63265ddf628da44d7/posthog/src/main/java/com/posthog/PostHog.kt#L615-L620
this line should only be executed if userProperties or userPropertiesSetOnce have changed from previous calls.
we should cache userProperties and userPropertiesSetOnce in the cachePreferences to be able to compare