Enhance widget customization

[IguJl15]

Increase customization of the Country Code Dropdown Form Field:

Fixes my own issue/suggestion

• New short-hand property to show or hide the flag icon on both dropdown value and list (inspired on #156)

• Added some dropdown field style properties to enhance customization

• Turn onCountrySelected property nullable, allowing the input to be disabled. Usage:

  CountryDropdown(
    onCountrySelected: null, // Dropdown button disabled
    initialCountryData: PhoneCodes.getPhoneCountryDataByCountryCode(
      'RU',
    ),
  )
  

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Disabled

Screencast from 2024-07-17 23-40-54.webm

Show flag

Screencast from 2024-07-17 23-42-49.webm