Include missing attributes on types.Unknown

[TristonianJones]

Feature request checklist

• There are no issues that match the desired change
• The change is large enough it can't be addressed with a simple Pull Request
• If this is a bug, please file a Bug Report.

Change
Currently, the types.Unknown value is a simple type alias to a list of integers; however,
the unknown response is significantly richer in C++, as it includes the list of missing
attributes in addition to the expression ids which were unknown.

Note: this will be a breaking API change for users of types.Unknown; however, it will
make the unknown responses significantly more usable than they are today.

Alternatives considered
It's possible to reconstruct the missing attributes and calls post-evaluation; however,
doing so accurately requires state tracking which is a significantly expensive overhead
and likely not worth the cost in most cases.