Add CAST expression support to parser

[nssalian]

Closes #198

Picks up where #209 left off with @Fokko's feedback addressed. Thanks @jayceslesar for the original work.

Rationale for this change

Adds CAST(column AS type) parsing that maps to Iceberg transforms (date → DayTransform, year → YearTransform, month → MonthTransform, hour → HourTransform). Also implements BoundTransform.ref() and eval() — both were missing abstract methods required by BoundTerm. Introduces UnboundTransform with bind() that validates via can_transform().

Are these changes tested?

Yes. 11 new tests covering all transform types, case insensitivity, nested fields, unsupported types, comparison operators, and boolean composition.

Are there any user-facing changes?

No.

[nssalian]

@Fokko @geruh @kevinjqliu PTAL

[Fokko]

It looks like we've already copied this logic multiple times, should we consolidate this:

iceberg-python/pyiceberg/expressions/parser.py

Lines 161 to 192 in d008a04

	@left_ref.set_parse_action
	def _(result: ParseResults) -> BooleanExpression:
	if result.op == "<":
	return LessThan(result.column, result.literal)
	elif result.op == "<=":
	return LessThanOrEqual(result.column, result.literal)
	elif result.op == ">":
	return GreaterThan(result.column, result.literal)
	elif result.op == ">=":
	return GreaterThanOrEqual(result.column, result.literal)
	if result.op in ("=", "=="):
	return EqualTo(result.column, result.literal)
	if result.op in ("!=", "<>"):
	return NotEqualTo(result.column, result.literal)
	raise ValueError(f"Unsupported operation type: {result.op}")
	@right_ref.set_parse_action
	def _(result: ParseResults) -> BooleanExpression:
	if result.op == "<":
	return GreaterThan(result.column, result.literal)
	elif result.op == "<=":
	return GreaterThanOrEqual(result.column, result.literal)
	elif result.op == ">":
	return LessThan(result.column, result.literal)
	elif result.op == ">=":
	return LessThanOrEqual(result.column, result.literal)
	elif result.op in ("=", "=="):
	return EqualTo(result.column, result.literal)
	elif result.op in ("!=", "<>"):
	return NotEqualTo(result.column, result.literal)
	raise ValueError(f"Unsupported operation type: {result.op}")

[Fokko]

For testing, I think it would be good to have an integration test as well. The StringLiteral should be converted to a DateLiteral when bound to the transform.