Spark 4.2: Implement SupportsSchemaEvolution for automatic schema evolution

[rahulsmahadev]

Implements Spark 4.2's SupportsSchemaEvolution mix-in on Iceberg's SparkTable, so Iceberg participates in DSv2 automatic schema evolution during writes.

During automatic schema evolution Spark passes each candidate column change to supportsColumnChange(TableChange.ColumnChange) and skips the ones the source reports as unsupported. SparkTable now reports support consistent with what Iceberg's schema update actually applies:

• add column: only when nullable, without a default, and with a Spark type Iceberg can convert;
• update type: only when the new type is a primitive and an allowed Iceberg type promotion;
• update nullability: only when relaxing to nullable;
• delete column: rejected for identifier fields and for any field whose subtree contains an identifier field (matching SchemaUpdate), and for unknown fields;
• rename, comment, and position changes are supported.

SparkTable already advertises TableCapability.AUTOMATIC_SCHEMA_EVOLUTION (gated by the write.spark.auto-schema-evolution.enabled table property), which is what makes Spark consult this hook. A unit test is added in TestSparkTable.