Implement 'diff' functionality in SqlGenerator.ts

[coderabbitai[bot]]

A TODO comment in indicates that the 'diff' functionality is missing. This issue tracks the implementation of this functionality as discussed in PR https://github.com/eggjs/tegg/pull/192 and the specific comment https://github.com/eggjs/tegg/pull/192#discussion_r1532284028.

Please ensure to review the context of the PR and the discussion to understand the requirements and expectations for this implementation.