Add methods for modules to safely get/set data using storage layer.

Problem - Full storage layer access to modules allows them to modify metadata of tkey shares.

Potential solution:-
- Add set and get metadata APIs on core.ts. They will serve as middleware for modules to set data.

Start by reading core.ts and tracing how modules currently access the storage layer and tkey share metadata. Define the scope of the get/set metadata APIs and verify that module data access is routed through them without exposing full storage-layer access.

Written by the indexing model from the issue text.