fix(bindings): fix distance return types; add + shift alias for tstzset/tstzspan+interval

[estebanzimanyi]

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Summary

• Integer and date span distance operators were registered with INTEGER/BIGINT return types but the C++ executor wrote DOUBLE values → INTERNAL error at runtime. Fixed by using BinaryExecutor<..., int32_t/int64_t> with DatumGetInt32/Int64.
• tstzspan distance was registered as DOUBLE but callers expect INTERVAL (e.g. 2 days, not 172800.0). Fixed to return interval_t via SecsToInterval.
• Added + and shift aliases for tstzset/tstzspan + interval (DuckDB cannot define custom infix operators, so + is registered as a named function alias).

Test plan

• intspan '[1,3]' <-> intspan '[5,7]' returns 2 (INTEGER)
• datespan '[2000-01-01,2000-01-03]' <-> datespan '[2000-01-10,2000-01-15]' returns correct day count
• tstzspan '[2000-01-01,2000-01-03]' <-> tstzspan '[2000-01-10,2000-01-15]' returns 7 days INTERVAL

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[estebanzimanyi]

Consolidation note — overlaps with #106

See #106 comment for the full overlap matrix. Both PRs touch distance return types in src/temporal/span_functions.cpp with different helper names (SecondsToInterval here vs SecsToInterval in this PR's time_util.hpp) and different Distance_span_span shapes (this PR's per-type switch is the more complete one).

Maintainer call which to keep, which to fold.

[estebanzimanyi]

Superseded by the consolidated parity batch at #126 (feat/parity-additions-batch, commit c53a482) and the final 14-name closing batch at #130 (feat/parity-final-batch, commit 22c64c0).

The work on this branch — distance return types + tstzset/tstzspan+interval alias — is now part of the squashed addressable-parity surface that brings MobilityDuck to 943/943 = 100.0% of the active addressable temporal + geo names. Closing to keep the review queue focused on the three active stacked PRs (#126 → #130 → #129) and the four orthogonal PRs (#105 / #111 / #113 / #117 / #120).

Branch fix/span-distance-return-types is left intact; reopen if a regression appears in the squashed batches and the originals turn out to carry a unique fix.