Adding ICAO 9303 Nationality codes

[JSenart]

According to Norm ICAO 9303, nationality codes are defined for machine readable documents with these specifications
https://www.icao.int/publications/Documents/9303_p3_cons_en.pdf
These are the codes stated in official documents MRZs.

Added them here as this is the most prominent country picker package that I would like to use for country picking in tourist forms to be submitted to immigration services.

[JSenart]

I can also remove the codes from subject countries and create a new mode of country picker where only the Nationalities are shown (There are less nationalities than countries). I wanted however to make minimal changes to your repo and your approval before making such adition.

[Daniel-Ioannou]

@JSenart First thank you for your contribution.

I originally approved and merged this pull request but after I run the example project I saw that the package through an exception.

At Country mode you implemented ICAO 9303 field as not nullable but at country_codes.dart you did not provide the ICAO 9303 for all countries.

Please fix the errors and reopen the pull request.

type 'Null' is not a subtype of type 'String'

The relevant error-causing widget was: 
  Container Container:file:flutter/package/country_picker/lib/src/country_list_bottom_sheet.dart:74:10
When the exception was thrown, this was the stack: 
#0      new Country.from (package:country_picker/src/country.dart:89:18)
#1      new CountryService.<anonymous closure> (package:country_picker/src/country_service.dart:11:51)
#2      MappedListIterable.elementAt (dart:_internal/iterable.dart:413:31)
#3      ListIterator.moveNext (dart:_internal/iterable.dart:342:26)
#4      new _GrowableList._ofEfficientLengthIterable (dart:core-patch/growable_array.dart:189:27)
#5      new _GrowableList.of (dart:core-patch/growable_array.dart:150:28)
#6      new List.of (dart:core-patch/array_patch.dart:51:28)
#7      ListIterable.toList (dart:_internal/iterable.dart:213:44)