Expose ``KeyValueStack`` to extensions to interact with plugin kv handles

[fakuivan]

Exposes methods via the sm interface for creating and reading kv handles and adds a new interface for KeyValueStack. This should allow extensions to provide kv handles to plugins and read them when necessary.

[psychonic]

Do we really need a way to expose the whole KeyValueStack, rather than just a way to create the handle from the KeyValues*? @asherkin

[fakuivan]

Any updates on this?