feat: support MySQL ALTER TABLE ... PARTITION BY syntax

[minleejae]

Description

This PR adds support for the MySQL ALTER TABLE ... PARTITION BY syntax in the JSQLParser library. The implementation includes parsing and deparsing of PARTITION BY clauses, handling both partition expressions and partition columns.

Changes

• Updated AlterExpression class to handle partition expressions and partition columns.
• Modified toString method in AlterExpression to correctly format PARTITION BY clauses.
• Added test cases in AlterTest to verify the correct parsing and deparsing of ALTER TABLE ... PARTITION BY statements.

[manticore-projects]

That's a really nice addition, thank you for your contribution!