feat(evolution): support v3 column default values in UpdateSchema (3/4)

[huan233usc]

What

Part 3 of 4 of Iceberg v3 column default-value support (POC #731), built on the
schema-layer support merged in #746. Independent of the read-path PRs (#792 and
the Avro follow-up).

Adds default-value handling to UpdateSchema (schema evolution).

Changes

• AddColumn / AddRequiredColumn take an optional default_value. When
  provided it is set as both the column's initial-default and write-default.
  A non-null default also lets a required column be added without
  AllowIncompatibleChanges() — rows written before the change read the default
  instead of null.
• UpdateColumnDefault(name, default) (new) sets, or clears with
  std::nullopt, a column's write-default; the initial-default is fixed when
  the column is added.
• Defaults are cast to the column type (rejecting uncastable or out-of-range
  values) and preserved across rename / doc / type-promotion updates and
  nested field-id reassignment.
• RequireColumn may now mark a column that was added with a default required.

The SchemaField constructor stores defaults verbatim (it does not coerce
them), so the cast/promotion is performed explicitly at each evolution site —
the same effect as Java, where NestedField's constructor runs castDefault.
Same-scale decimal precision widening is handled directly (the unscaled value is
unchanged), since Literal::CastTo does not cast between decimal types.

Tests

13 cases in update_schema_test.cc: add optional/required/nested column with a
default, mismatched/narrowing rejection, UpdateColumnDefault
(set / clear / cast-to-type / pre-existing column), require-after-default, and
preservation across doc updates and type promotion — including same-scale
decimal precision promotion.

Stack

1. feat(schema): represent, serialize and validate v3 column default values (1/4) #746 — schema: represent / serialize / validate (merged)
2. feat(parquet): apply column default values when reading missing fields (2/4) #792 — read path: Parquet
3. read path: Avro (follows)
4. this PR — schema evolution: addColumn / updateColumnDefault