Change CanChangeState::shouldCopyValue to only match bson values.

SBE deals with 2 value sources:

• internal: all values created and managed by SBE. Those values are pointer stable, and don't require special handling during yielding save/restore.
• external: values that are coming from outside of SBE (like bson returned by a storage cursors). Those values might not have stable pointers and in such case require special handling (copy) during yielding save/restore.

We should limit CanChangeState::shouldCopyValue to only match the external values that have unstable pointers.