Added operations on time stamps

[liuzicheng1987]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @liuzicheng1987, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the sqlgen library by introducing robust support for timestamp and duration operations. It allows users to perform date/time arithmetic and extract specific components from timestamp columns in a type-safe C++ manner, which is then correctly transpiled into appropriate SQL for both PostgreSQL and SQLite databases. This expands the library's capabilities for handling time-series data and complex date-based queries.

Highlights

• Timestamp and Duration Operations: Introduced comprehensive support for arithmetic operations (+, -) on timestamp columns using std::chrono::duration types, and added functions for extracting date/time components (year, month, day, hour, minute, second, weekday) and calculating differences (days_between, unixepoch).
• Type System Enhancements: Added Date and DateTime type aliases, along with new type traits (is_duration, is_timestamp) and updated type deduction logic to correctly handle timestamp and duration types throughout the sqlgen framework.
• Database Compatibility: Implemented specific SQL generation logic for the new timestamp and duration operations, ensuring compatibility and correct translation for both PostgreSQL and SQLite databases.
• New Test Coverage: Added new test cases for both PostgreSQL and SQLite to validate the functionality of timestamp arithmetic and date/time component extraction in SELECT and WHERE clauses.

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[gemini-code-assist]

Code Review

This pull request introduces significant functionality for handling timestamp and duration operations. I've identified a few critical issues related to correctness that need to be addressed, particularly in the transpilation::Operation logic and the PostgreSQL SQL generation for unixepoch. I've also included some suggestions to improve maintainability and fix issues in the new tests.