poc: extend exists/2 to traverse embedded array attributes

[nallwhy]

poc of #2698

[zachdaniel]

Looking at the Ash PR it generally looks good! Haven't looked too deeply at the others. However, one thing I'd like to see is a test proving that the relationship path referencing works with this. There are example tests in the policy tests that exercise these over exists etc.

[nallwhy]

Good call — added a test covering exactly that pattern, mirroring parent_test.exs:164 (where parent/1 traverses a belongs_to relationship into a related row's attribute) but with an embedded-array target on the outside:

exists(options, name == parent(company.name))