Add a trait to URef to make them shareable but not forwardable.

BIG breaking change.---> This new trait will enable whitelists to verify callers, especially when a contract calls another.
New property "sharing" set on creation - New default: "limited sharable" or "uref not shareable".
TODO: Need to define how this is limited. Maybe: share N times - implemented via a counter N on the number of shares
controlled by economics: Existing URefs are infinitely sharable - Infinite is possible, but very expensive.
URefs will be shared at the same permission level or less.

Enhancement: Consider: sharing will share at the least permission but can be increased (basically a reverse of what exists today).

The issue proposes a breaking-change trait and a new sharing property for URefs, but names no files, tests, or implementation entry points. Start by clarifying the sharing limits, permission rules, defaults, and economic model described in the TODOs. Done requires an agreed specification for shareability, non-forwardability, and treatment of existing URefs.

Written by the indexing model from the issue text.