Optimize merging of partial case expression results (#18152)

## Which issue does this PR close?

- Improvement in the context of
#18075
- Continues on #17898

## Rationale for this change

Case evaluation currently uses `PhysicalExpr::evaluate_selection` for
each branch of the case expression. This implementation is fine, but
because `evaluate_selection` is not specific to the `case` logic we're
missing some optimisation opportunities. The main consequence is that
too much work is being done filtering record batches and scattering
results. This PR introduces specialised filtering logic and result
interleaving for case.

A more detailed description and diagrams are available at
#18075 (comment)

## What changes are included in this PR?

Rewrite the `case_when_no_expr` and `case_when_with_expr` evaluation
loops to avoid as much unnecessary work as possible. In particular the
remaining rows to be evaluated are retained across loop iterations. This
allows the record batch that needs to be filtered to shrink as the loop
is being evaluated which reduces the number of rows that needs to be
refiltered. If a when predicate does not match any rows at all,
filtering is avoided entirely.

The final result is also not merged every loop iteration. Instead an
index vector is constructed which is used to compose the final result
once using a custom 'multi zip'/'interleave' like operation.

## Are these changes tested?

Covered by existing unit tests and SLTs

## Are there any user-facing changes?

No

Author: pepijnve