Add lookup by `path` mode to `InstrospectCommand`

Current introspection implementation support only getting a value by a key or a stateId for a singleton. In some cases we may know the path of the value, and we need to see the value.

By path we'll get bytes representing `StateValue`. This object contains `stateId` that we can use to lookup a codec for a value this object contains.

Start at InstrospectCommand and trace how key or singleton stateId lookups obtain and decode a value. Use the path to obtain the StateValue, then use its stateId to select the codec; done means introspection can return the value for a supplied path.

Written by the indexing model from the issue text.