added support of external keyword for field builder

[Snehal-Singh174]

• Added support of external keyword for FieldBuilder
• Issue #409
• 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;
    '''),
  );
  });

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[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