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ARROW-13987: [C++] Support nested field refs

This implements nested field refs in C++ only, using a SmallVector to hold the FieldPath. This only lets us extract from a struct (I'm not so sure it makes sense for other types?).

The JIRA also requests being able to extract a field as an expression. I think this could be done by implementing a small kernel that we could call. (Or otherwise I think we'd have to add a new case to the Expression variant, which maybe isn't a big deal.) If that sounds reasonable it can be added here.

A microbenchmark was added to see if this impacts the common case of a non-nested field ref. On my local machine it does not appear to (~10-20ns difference).

Closes #11466 from lidavidm/arrow-13987

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>