Allow state shape with new optional fields

[mhofman]

What is the Problem Being Solved?

Exo/kinds using stateShape cannot change their shape when upgrading. #7407 describes a way to do schema migration, but a simpler approach is possible for state shapes that are "compatible" as described in #7407 (comment): existing fields must retain identical shapes (including exact identity of any remotable reference), and new fields are allowed only if they're optional (allow undefined values)

Description of the Design

when liveslots checks the compatibility of the state shape, iterate over fields of the shape, assert that any previously existing fields are the same pattern, and that any new field accepts the undefined value.

When reading state data from the vatStore, accept a missing entry as an undefined value.

When writing state data, handle a previously missing entry. Consider "deleting" the prop's entry if the value is undefined (cheap compression)

Security Considerations

None

Scaling Considerations

None

Test Plan

Some fake liveslots based test which triggers kind redefinition logic.

Upgrade Considerations

Requires a new liveslots, and thus a chain software upgrade