Allow Usage of ISO Codes Instead of Country Logos in the Leading of Input Field

[Rohan-ManekTech]

Currently, the "intl_phone_number_input" Flutter package utilizes country logos as leading icons in the input field to indicate the selected country code. While this feature provides visual appeal, it may not be suitable for all use cases, especially when a more minimalistic or standardized approach is preferred.

This issue proposes the addition of a new configuration option that allows developers to use ISO codes instead of country logos in the leading of input field. By providing this option, developers can have more flexibility and control over the appearance of the input field and better align it with the design requirements of their Flutter applications.

The proposed configuration option should enable developers to:

1. Choose between displaying country logos or ISO codes in the leading of the input field.
2. Customize the appearance of the ISO code (e.g., font size, font color) to match the app's design theme.
3. You can access the corresponding ISO code programmatically for further use in the application logic.
   The implementation should be backward compatible and not introduce breaking changes for existing package users who wish to continue using country logos.

This enhancement would make "intl_phone_number_input" more versatile and cater to a broader range of Flutter app development needs, ensuring a consistent and coherent user experience across different projects.