feat(bindings): scalar extent overloads (integer/bigint/float/date/timestamptz)

[estebanzimanyi]

Summary

Adds 5 scalar overloads to the `extent()` `AggregateFunctionSet`, matching MobilityDB's `CREATE AGGREGATE extent(integer)` / `(bigint)` / `(float)` / `(date)` / `(timestamptz)` registrations from `015_span_aggfuncs.in.sql`:

Overload	Returns
`extent(integer)`	`intspan`
`extent(bigint)`	`bigintspan`
`extent(float)`	`floatspan` (DuckDB `DOUBLE`)
`extent(date)`	`datespan`
`extent(timestamptz)`	`tstzspan`

```sql
SELECT extent(v) FROM (VALUES (1), (5), (3)) tt(v);
-- [1, 6)

SELECT extent(v) FROM (VALUES (DATE '2000-01-01'), (DATE '2000-01-05')) tt(v);
-- [2000-01-01, 2000-01-06)
```

Implementation

Each subtype gets its own functor (`IntExtentFunction`, `BigintExtentFunction`, `FloatExtentFunction`, `DateExtentFunction`, `TimestamptzExtentFunction`) that bridges to the matching MEOS `*_extent_transfn` (`int_extent_transfn`, `bigint_extent_transfn`, etc.). All share the existing `SpanExtentState` and `SpanExtentBase` (Initialize / Combine / Finalize) from PR #47.

Epoch conversion: `date` and `timestamptz` inputs use DuckDB's 1970 epoch; MEOS uses PostgreSQL's 2000 epoch. The functors convert via the existing `time_util.hpp` helpers (`ToMeosDate` / `DuckDBToMeosTimestamp`). The output Span is stored in MEOS-native epoch form, which round-trips correctly through the existing span in/out functions.

Stacked on

• feat(bindings): aggregate function infrastructure — extent(span) for all 5 span types #47 → feat(bindings): extend extent(*) to spanset and set surfaces #48 → feat(bindings): extent(tnumber) → tbox + bundle extent overloads in one set #49 → feat(bindings): tCount(temporal) aggregate — first skiplist-backed aggregate #50 → feat(bindings): tAnd / tOr / tagg_min / tagg_max / tagg_sum aggregates #52 → feat(bindings): spanUnion(span | spanset) → spanset aggregate #53 → feat(bindings): tagg_min(ttext) / tagg_max(ttext) overloads #54 (aggregate stack)

Test plan

• All 5 scalar input types produce correct extent
• NULL handling (mid-run NULL ignored)
• Date / timestamptz round-trip correctly across the epoch boundary
• Existing span/spanset/set/tnumber overloads still work (regression)

[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.