Refactor: split CREATE TABLE and ALTER TABLE grammar into smaller productions

Extract shared helpers reused across both statements:
- ReferentialActionsOnIndex, CheckConstraintSpec, ForeignKeySpec
- AlterExpressionUsingIndex, AlterExpressionConstraintTail

Split monolithic productions into focused sub-productions:
- CreateTableConstraint: constraint dispatch inside CREATE TABLE (...)
- AlterExpressionAddConstraint: CONSTRAINT clause in ALTER TABLE
- AlterExpressionDrop: all DROP variants
- AlterExpressionPartitionOp: partition maintenance operations

CreateTable shrinks from 211 to 86 lines, AlterExpression from 754 to 477.
No changes to Java model classes or public API.

Signed-off-by: Andreas Reichel <andreas@manticore-projects.com>
Signed-off-by: manticore-projects <andreas@manticore-projects.com>

Author: manticore-projects