[C++] Create a fetch node based on a batch index property

[westonpace]

Describe the enhancement requested

This node will eventually be usable mid-plan allowing us to get rid of the concept of a TopKSink and also to add Substrait consumer support for fetch.

This feature introduces the idea of an "ExecBatch::index" which is described more fully in https://docs.google.com/document/d/1MfVE9td9D4n5y-PTn66kk4-9xG7feXs1zSFf-qxQgPs/edit

This feature is part of a larger set of features and this node will not be very useful on its own until some of those features are added (currently, all nodes except table_source will either fail to set the index or will reset it back to -1 and the fetch node will fail). See #32991 for the larger roadmap.

Component(s)

C++