Migrate subscriber attributes to a UserDefaults key per attribute

[vegaro]

Based off #5959

Refactored from a grouped dictionary format to individual keys per attribute:

• Before: com.revenuecat.userdefaults.subscriberAttributes → { "userID1": { "height": {...}, "weight": {...} } }
• After: com.revenuecat.userdefaults.subscriberAttributes.userID1.height → { attribute data }

This eliminates read-modify-write (RMW) operations entirely since each attribute write is now a single atomic key write, removing the need for locking.

A migration function (migrateGroupedToIndividualKeys) handles upgrading existing data from the old format.

[RevenueCat-Danger-Bot]

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