Fix issue 2089: Enhance MySQL CONVERT Statement Parsing

[minleejae]

Summary
This PR fixes and enhances the parsing functionality for MySQL CONVERT statements in ALTER TABLE clauses in JSQLParser. It now supports a wider variety of CONVERT syntax.

Changes
Updated AlterOperation enum to include CONVERT.
Modified the parser to correctly identify and handle the CONVERT clause, including optional COLLATE specifications.

Enhanced the parser to support the following ALTER TABLE syntax:

ALTER TABLE tbl_name
    | [DEFAULT] CHARACTER SET [=] charset_name [COLLATE [=] collation_name]
    | CONVERT TO CHARACTER SET charset_name [COLLATE collation_name]