Provide an interface for setting the unutilised storage elements of a token account

[kantp]

Proposed feature

Implement an interface for setting all unutilised storage elements under the token account guarded by customisable permissions.

Why

This feature would unlock many workflows, for example: users can store data such as a nickname, on-chain application preferences, statistics guarded by proof-only updates and soul-bounded non-updateable data. (examples just to visualise my point). Generally gives zkapp developer a user specific storage pod.