feat: add support for #[builder(into)] attribute

[tomoikey]

Closes #8

Summary

• Add support for #[builder(into)] attribute to automatically convert setter parameters using Into trait
• Improves ergonomics by allowing string literals and other types to be passed directly to setters that expect String

Changes

• Added into_flag field to InputField struct to track #[builder(into)] attribute
• Modified setter generation to use impl Into<T> parameter type when into flag is present
• Updated field info tuple to include into flag information
• Added test case demonstrating the functionality

Relates to

#8