Add camel case to snake case conversion

[cacharle]

Fixes part of #585

[gemini-code-assist]

Summary of Changes

Hello @cacharle, 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 rfl library by adding the capability to convert field names from camelCase to snake_case, addressing a common requirement in data serialization. This new feature is integrated through a new CamelCaseToSnakeCase struct and is supported by a refactored and more unified internal case transformation mechanism. The changes improve flexibility for users dealing with different naming conventions and are thoroughly tested to ensure correctness and compatibility with existing features like field renaming.

Highlights

• New Feature: CamelCaseToSnakeCase: Introduced a new utility rfl::CamelCaseToSnakeCase that automatically transforms struct field names from camelCase to snake_case during serialization and deserialization. This complements existing case conversion functionalities.
• Internal Refactoring of Case Transformation: The internal case transformation logic has been refactored. The transform_snake_case.hpp header was renamed to transform_case.hpp and now houses a new transform_camel_case compile-time function, along with a is_upper helper, centralizing case conversion utilities.
• New Test Case: A dedicated test file test_camel_case_to_snake_case_rename.cpp has been added to validate the new CamelCaseToSnakeCase functionality, ensuring it correctly handles field names and respects explicit renames using rfl::Rename.
• Minor Optimization: An optimization was added to the to_camel_case function in strings.cpp by reserving string capacity, potentially improving performance for string manipulations.

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[gemini-code-assist]

Code Review

This pull request introduces a CamelCaseToSnakeCase processor, which is a valuable addition. The refactoring to consolidate case transformation logic into a shared transform_case.hpp file is also a good improvement. I've identified a bug in the new transformation logic that results in incorrect conversions for fields starting with an uppercase letter, along with a couple of minor issues. My review includes specific suggestions to address these points.

[liuzicheng1987]

Looks good to me.

[liuzicheng1987]

@cacharle thanks for the contribution. I am merging.

[cacharle]

Just realized I forgot to add anything to the doc