arrow

GH-18818: [R] Create a field ref to a field in a struct (#19706)

This PR implements `$.Expression` and `[[.Expression` methods, such that if the Expression is a FieldRef, it returns a nested FieldRef. This required revising some assumptions in a few places, particularly that if an Expression is a FieldRef, it has a `name`, and that all FieldRefs correspond to a Field in a Schema. In the case where the Expression is not a FieldRef, it will create an Expression call to `struct_field` to extract the field, iff the Expression has a knowable `type`, the type is `StructType`, and the field name exists in the struct. 

Things not done because they weren't needed to get this working:

  * `Expression$field_ref()` take a vector to construct a nested ref
  * Method to return vector of nested components of a field ref in R

Next steps for future PRs:

* Wrap this in [tidyr::unpack()](https://tidyr.tidyverse.org/reference/pack.html) method (but unfortunately, unpack() is not a generic)
* https://github.com/apache/arrow/issues/33756
* https://github.com/apache/arrow/issues/33757
* https://github.com/apache/arrow/issues/33760

* Closes: #18818

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>