build-apple: Organize headers into a sub-directory for XCFramework

[xiguagua]

Description:

This PR refactors the Apple XCFramework build script to organize public headers into a sub-directory named HevSocks5Tunnel within the include folder.

Key benefits:

• Standardization: Aligns with Apple's standard framework header structure.
• Collision Prevention: Avoids potential header filename conflicts when integrated into larger projects.

[heiher]

cc @heiby @yiguodev

[heiher]

Thanks!