arrow

GH-41323: [R] Redo how summarize() evaluates expressions (#41223)

### Rationale for this change

Previously, the NSE expression handling in `summarize()` worked
differently from filter/mutate/etc. Among the implications, it would not
have been possible to define bindings for other aggregation functions
that can be translated into some combination of supported aggregations,
such as `weighted.mean()`.

### What changes are included in this PR?

* Expressions in `summarize()` can now be evaluated with "regular"
`arrow_eval()`. Aggregation bindings stick the contents of the
aggregation data they previously returned into an `..aggregations` list
that lives in an enclosing environment, and then return a FieldRef
pointing to that. This makes the code in e.g. `summarize_eval()` a
little harder to follow, since it's grabbing and pointing to objects out
of its immediate scope, but I've tried to comment thoroughly and am
happy to add more.
* `arrow_eval()` inspects the expression it receives for any functions
that are not in the NSE mask and not in some other package's namespace
(i.e. hopefully just user functions) and inserts them into the NSE mask,
setting the enclosing environment for that copy of the function to be
the mask, so that if the function calls other functions that we do have
bindings for, the bindings get called. This is the approach I suggested
back in
https://github.com/apache/arrow/issues/29667#issuecomment-1378049226,
and it is what fixes #29667 and #40938.

### Are these changes tested?

Existing tests, which are pretty comprehensive, pass. But it would be
good to try to be more evil in manual testing with the user-defined R
function support.

### Are there any user-facing changes?

Yes.

* GitHub Issue: #41323