Refactor: Main functions as hidden friends

[liuzicheng1987]

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[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 implements a refactoring of the SQL query building pipeline. Specifically, the operator| overloads that enable chaining operations like where, group_by, order_by, limit, and to are redefined. They are moved from being global or namespace-level free functions into the class definitions themselves as friend functions. This is a standard C++ refactoring technique known as the 'hidden friend' idiom, which can improve encapsulation and the discoverability of operators via ADL.

Highlights

• Refactoring Operator Overloads: The operator| overloads used for chaining SQL query components (like where, group_by, order_by, limit, to) have been refactored.
• Hidden Friend Idiom: The operator| definitions have been moved from being free functions to being defined as friend functions inside the relevant structs (CreateIndex, DeleteFrom, Read, SelectFrom, Update). This leverages the 'hidden friend' idiom.
• Improved Code Structure: This change improves code organization by placing the operator definitions closer to the types they operate on and potentially aids Argument-Dependent Lookup (ADL).

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Code Review

The code changes introduce hidden friend operator| overloads for fluent interface building of SQL queries. The changes improve code organization and readability.