[Feature Request]: optionally persist custom attributes

[williazz]

Currently, we are assuming that custom attributes are static and loaded by site operators on every hard refresh / page load via addSessionAttributes. However, some operators dynamically change custom attributes based on user behavior, and want these changes to be persisted across hard refreshes. In that particular use case, the desired custom attributes are lost. To resolve this, we should add a feature where custom attributes can be written to and loaded from the Local Storage API.