Support variadic tail slots in #[array_slots] macro

[joseph-isaacs]

Rationale for this change

The #[array_slots] macro currently supports fixed-size slot layouts with required (ArrayRef) and optional (Option<ArrayRef>) fields. However, several encodings like Chunked, Struct, and Union have variable-length slot runs (e.g., [chunk_offsets, chunks...]) that cannot be cleanly expressed with the current macro.

This change extends the macro to support a variadic tail — a final field of type Vec<ArrayRef> — enabling these encodings to use the macro-generated slot accessors and constants instead of hand-written slot index management.

What changes are included in this PR?

Macro enhancements (vortex-array-macros/src/lib.rs)

• Added SlotFieldType::VariadicTail variant to recognize Vec<ArrayRef> fields
• Validation ensures variadic tails only appear as the final field
• Split slot field processing into fixed_specs and optional tail_spec
• Generated different constant sets for variadic layouts:
  • FIXED_COUNT: number of fixed slots
  • FIXED_NAMES: names of fixed slots
  • {FIELD}_OFFSET: offset where the tail begins
  • slot_name(idx) function: returns slot names like "chunks[3]" for tail slots
• Updated from_slots() and into_slots() to handle tail draining/extending
• View struct and ext trait accessors for tail fields now return SlotSlice<'a> instead of Vec<ArrayRef>