AddFiles: SchemaDelta classifies what a file schema needs from the table (#40062)

* AddFiles: SchemaDelta classifies what a file schema needs from the table

The classifier behind the options. Iceberg's unionByNameWith has no
knobs: it adds, relaxes and promotes in one go, or throws. To honour
ALLOW_FIELD_ADDITION / ALLOW_FIELD_RELAXATION / ALLOW_TYPE_PROMOTION
separately, SchemaDelta.classify(table, fileSchema) applies the union
on a throwaway UpdateSchema (apply(), never commit()), diffs the result
against the current table schema, and labels every change:

- FIELD_ADDITION: a field id present only after the union.
- FIELD_RELAXATION: required before, optional after; or a required
  table column with no counterpart in the file at all (see below).
- TYPE_PROMOTION: same id, wider primitive type after.
- CONFLICT: anything else. The union throwing (ValidationException or
  IllegalArgumentException, e.g. int column vs string file column, a
  dotted or empty file column name), a field removed by the union
  (cannot happen with unionByName but is refused rather than trusted),
  a struct where a primitive was, a doc string or default changing, a
  promotion Iceberg would not allow (TypeUtil.isPromotionAllowed guard,
  so a bad union result is never staged as a "promotion").

The diff is keyed by field id and walks fields attribute by attribute
(name, optionality, type kind, doc, defaults), so an attribute the union
silently changes is reported rather than committed unnoticed. Changes
are listed in a deterministic order (unquoted path) with quoted names in
messages so a reviewer can find them in the schema.

Absence rule: a required table column that the file lacks is a
FIELD_RELAXATION, not a pass. Registering such a file would put nulls
in a required column for every reader; the fix is to relax the column
explicitly (the commit side stages makeColumnOptional for exactly the
paths absentRequiredPaths() reports). The walk descends through structs
whose parent is present, through list elements and map values (paths
use "element" and "value", which makeColumnOptional accepts); an absent
struct is itself the relaxation, its children are not listed
separately; map keys are required by definition and skipped.

Pins: Change.allowedBy(config) refuses a relaxation of a pinned path
even when ALLOW_FIELD_RELAXATION is set, and disallowedReason(config)
names it, so "id is pinned" shows up as the reason rather than a
generic "relaxation not allowed".

* move pins to class

* pins

* docstring

* Fix docstring

* tests

* split file

* split

* map keys

* track test