Core, Spark: Introduce shared MultiColumnTerm to de-duplicate Z-order and Hilbert clustering

[GGraziadei]

Closes #17657.

Follow-up to #16827, which deliberately mirrored the Z-order surface to keep that diff reviewable. This PR is the deferred de-duplication, along the lines @RussellSpitzer suggested in review ("we'll probably want a MultiColumnTerm or something like that in the future").

What changed

Core — new org.apache.iceberg.expressions.MultiColumnTerm, an immutable Term over an ordered list of column references. Zorder and Hilbert (previously identical apart from the class name) become two-line subclasses. Public constructors and refs() are unchanged; the parsed SQL grammar is untouched.

Spark 4.1 — SparkZOrderFileRewriteRunner and SparkHilbertFileRewriteRunner move onto a new abstract SparkCurveFileRewriteRunner, parameterised by the internal value column name, the exact per-curve error messages, and the combine step. The shared pieces — internal column schema/sort order, column validation (validZOrderColNames / validHilbertColNames were byte-for-byte the same logic), sortSchema, sortedDF, and the per-column ordered-bytes conversion — now live in one place. Each runner keeps only what is genuinely curve-specific: Z-order its two options and the interleave combine (~120 lines), Hilbert its fixed per-column bit width and the Hilbert-index combine (~75 lines). Adding a further curve is now a small, local change.

Dispatch — the duplicated zorder/hilbert branches in Spark3Util.toIcebergTerm share one reference-extraction helper, and RewriteDataFilesProcedure collapses its two per-curve term lists into a single List<MultiColumnTerm> (mixing curves is detected by distinct term classes; error messages unchanged).

What did not change

Engine-side behaviour is bit-for-bit identical, which the existing tests enforce: every pre-existing Z-order and Hilbert test passes unmodified (TestSparkFileRewriteRunners, the Z-order/Hilbert cases of TestRewriteDataFilesAction, TestRewriteDataFilesProcedure). The only test change is the new TestMultiColumnTerm in core. All user-facing error messages are preserved exactly. Spark 3.5/4.0 are untouched.

Out of scope

The wider "arbitrary function via Expressions" refactor discussed in #16827 review builds naturally on this abstraction but is split out per the issue, to keep this diff mechanical and easy to verify.

[GGraziadei]

Please consider GGraziadei#1 as the natural follow-up once this PR is merged. The idea is to extend the grammar to natively support transformations, improving the quality of Z-order and Hilbert clustering.

E.g.

CALL catalog.system.rewrite_data_files(
  table => 'db.sample', strategy => 'sort',
  sort_order => 'zorder(truncate(4, c2), days(c3))');