[fix] Invalid Fields structure

[nwithan8]

Description

Port of fix in .NET library to Java.

Fun fact: The Fields object in this library previously did not have any properties declared. This means there was never any deserialization issues, but also means that each Field was never reachable by end-users.

Testing

• Add unit test to confirm deserialization works as expected
  • The carrier accounts used in our existing unit tests do not have fields returned by the API, so the serialization issue was not previously caught.
  • Beyond that, the properties of Fields were not declared, so they were never deserialized.

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)