Flink: Avoid per-row field getter allocation in RowDataWrapper

[wombatu-kun]

RowDataWrapper adapts a Flink RowData to an Iceberg StructLike and sits on the hot path of every partitioned write (PartitionKey.partition), equality-delete key extraction, and range-shuffle sort-key computation. Its constructor pre-builds a getter for most types, but buildGetter returns null for the default case, which covers the most common primitive types: INT, BIGINT, BOOLEAN, FLOAT, DOUBLE, and DATE. For those fields, get(pos, javaClass) falls back to constructing a fresh Flink field getter on every access:

Object value = FlinkRowData.createFieldGetter(types[pos], pos).getFieldOrNull(rowData);

FlinkRowData.createFieldGetter allocates two lambdas per call (Flink's RowData.createFieldGetter plus the null-checking wrapper around it), so every row that is partitioned, keyed, or shuffled on a primitive column allocates two short-lived objects for each such field.

This pre-builds the fallback getter once in the constructor, the same way the wrapper already pre-builds getters for the handled types, so get no longer allocates. Behavior is unchanged: the raw Flink value is already the correct Iceberg representation for every default-case type, which is exactly what the previous fallback returned.

The change is identical across the supported Flink versions, so it is applied to v1.20, v2.0, and v2.1 in this PR.

Benchmark

JMH microbenchmark (JDK 17, -prof gc) that wraps a row and reads its fields. The schema has seven columns; readPrimitiveKey reads only the five primitive columns, representative of a typical partition or equality key.

Benchmark	Metric	Before	After	Delta
readPrimitiveKey	time	64.04 ns/op	45.31 ns/op	-29.3%
readPrimitiveKey	alloc	275.0 B/op	75.0 B/op	-72.7%
readAllFields	time	84.25 ns/op	81.37 ns/op	-3.4%
readAllFields	alloc	275.0 B/op	75.0 B/op	-72.7%

The remaining 75 B/op is return-value boxing that this change does not touch.

Existing TestRowDataWrapper coverage passes unchanged.