added support of external keyword for field builder

[Snehal-Singh174]

• Added support of external keyword for FieldBuilder
• Issue Add external key word for the FieldBuilder dart-lang/tools#1134
• Added test case for external keyword inside field_test

•   expect(
      Field((b) => b
        ..name = 'value'
        ..external = true
        ..type = refer('double')
        ..annotations.addAll([
          refer('Float').call([])
        ])),
      equalsDart(r'''
        @Float()
        external double value;
      '''),
    );
  });
  

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[natebosch]

I think the addition should be OK. Can you format with dart format to fix the CI error?

We will also need a CHANGELOG entry.

[Snehal-Singh174]

Done with the mentioned changes