feat(bindings): extend extent(*) to spanset and set surfaces

[estebanzimanyi]

Summary

Adds 10 new aggregate overloads to the `extent()` set introduced in #47:

• `extent() → ` for int/bigint/float/date/tstzspanset (5 overloads)
• `extent() → ` for int/bigint/float/date/tstzset (5 overloads — `textset` has no span equivalent so it's skipped)

```sql
SELECT extent(s) FROM (VALUES (intspanset '{[1, 3), [5, 7)}'), (intspanset '{[10, 12)}')) t(s);
-- [1, 12)

SELECT extent(s) FROM (VALUES (intset '{1, 3, 5}'), (intset '{10, 20}')) t(s);
-- [1, 21)

SELECT extent(s) FROM (VALUES (datespanset '{[2000-01-01, 2000-01-03)}'), (datespanset '{[2000-01-05, 2000-01-10)}')) t(s);
-- [2000-01-01, 2000-01-10)
```

Implementation

Adds `SpansetExtentFunction` and `SetExtentFunction` functors. Both reuse the shared `SpanExtentState` (`Span span + bool isset`) and `SpanExtentBase` (Initialize / Combine / Finalize). Only `Operation` differs — it dispatches to the corresponding MEOS transfn:

• `spanset_extent_transfn(state, ss)`
• `set_extent_transfn(state, s)`

These return a palloc'd `Span*` on first call (when state is NULL); the wrapper memcpy-into-state-and-frees, then expands the inline state in place on subsequent calls — same convention as the original `SpanExtentFunction`.

Stacked on

• feat(bindings): aggregate function infrastructure — extent(span) for all 5 span types #47 (AggregateFunction infrastructure + extent(span))

Test plan

• Smoke test all 3 surfaces (span / spanset / set) for all 5 base types
• NULL handling: mixed NULL + value works for each surface
• Existing test suite still passes locally

[estebanzimanyi]

Consolidated into #60 (aggregate cluster squash). The full chain of aggregate work is now reviewable as a single PR; this branch's commits are preserved in #60's history. Closing to minimize the review queue.