[Parquet][C++] Add experimental VECTOR repetition level for Arrow FixedSizeList

[rok]

This PR prototypes a new experimental Parquet repetition type, VECTOR, mainly for Arrow's FixedSizeList<T, N> as proposed in Option B here.

Today this is written through Parquet LIST, which means we pay repetition/definition-level costs for an inner shape that is already fixed. VECTOR stores the fixed length in the Parquet schema as vector_length and avoids the inner repetition level.

Writing FixedSizeList<T, N> as VECTOR is opt-in via ArrowWriterProperties::Builder::enable_experimental_vector_encoding() defaulting to LIST otherwise.

Implemented in this prototype

• Parquet schema/thrift addition of VECTOR
• Arrow schema conversion:
  • FixedSizeList -> VECTOR when explicitly enabled
  • VECTOR -> FixedSizeList
• required and nullable rows in VECTOR
• nullable primitive elements in VECTOR
• primitive type vectors
• limited composability proof of concept: FixedSizeList<struct<x: float, y: int32>, N>
• roundtrip/schema/path tests
• read/write/roundtrip benchmarks

Deferred for now:

• dictionary / non-PLAIN encodings
• statistics
• page index
• general nested VECTOR children (non-fixed will not be possible)

Compatibility

We expect non-VECTOR-aware readers to fail when encountering VECTOR repetition level.

Benchmark snapshot

All with FixedSizeList<float, {80,768,10k,100k}>.
Numbers below are from a local debug build, so directional only.

Non-nullable vector

Vector length	LIST write	VECTOR write	Write speedup	LIST read	VECTOR read	Read speedup	LIST roundtrip	VECTOR roundtrip	Roundtrip speedup
80	80.29k rows/s	13.58M rows/s	169x	108.94k rows/s	110.26M rows/s	1012x	50.25k rows/s	12.52M rows/s	249x
768	9.05k rows/s	1.50M rows/s	166x	13.18k rows/s	11.38M rows/s	863x	5.32k rows/s	1.23M rows/s	230x
10,000	701.42 rows/s	140.62k rows/s	200x	1.01k rows/s	871.62k rows/s	861x	411.96 rows/s	131.99k rows/s	320x
100,000	70.45 rows/s	16.08k rows/s	228x	101.06 rows/s	87.30k rows/s	864x	41.29 rows/s	12.51k rows/s	303x

Nullable vector

Vector length	LIST write	VECTOR write	Write speedup	LIST read	VECTOR read	Read speedup	LIST roundtrip	VECTOR roundtrip	Roundtrip speedup
80	144.77k rows/s	246.10k rows/s	1.7x	236.82k rows/s	1.07M rows/s	4.5x	95.86k rows/s	230.77k rows/s	2.4x
768	17.76k rows/s	71.77k rows/s	4.0x	25.98k rows/s	128.66k rows/s	5.0x	10.57k rows/s	46.66k rows/s	4.4x
10,000	1.39k rows/s	7.18k rows/s	5.2x	2.01k rows/s	10.39k rows/s	5.2x	819.23 rows/s	4.27k rows/s	5.2x
100,000	139.88 rows/s	707.80 rows/s	5.1x	201.00 rows/s	1.03k rows/s	5.1x	81.94 rows/s	421.25 rows/s	5.1x