[fix] Invalid CarrierFields structure

[nwithan8]

Description

This library was failing to deserialize a CarrierAccount from the API due to a failed deserialization of CarrierFields.

The previous assumption was that the API is returning "fields" in the JSON data for a CarrierAccount like so:

{
  "id": "ca_123",
  "object": "CarrierAccount",
  ...
  "fields": {
    "credentials": [
      {
        "key": "account_number",
        "visibility": "visible",
        "label": "DHL Account Number",
        "value": "123456"
      },
      {
        "key": "country",
        "visibility": "visible",
        "label": "Account Country Code (2 Letter)",
        "value": "US"
      }
    ]
  }
}

When in fact, the API is returning "fields" like so:

{
  "id": "ca_123",
  "object": "CarrierAccount",
  ...
  "fields": {
    "credentials": {
      "account_number": {
        "visibility": "visible",
        "label": "DHL Account Number",
        "value": "123456"
      },
      "country": {
        "visibility": "visible",
        "label": "Account Country Code (2 Letter)",
        "value": "US"
      }
    }
  }
}

Specifically, the "credentials" key under "fields" is not a list of dictionaries, but a dictionary of dictionaries, with the type of each credential field being the key in the key-value pair.

This PR updates the CarrierFields object structure to match the JSON schema.

Because the key for each type of credential field can by dynamic, users will have to access each credential field via a dictionary index:

string countryLabel = myCarrierAccount.Fields.Credentials["country"].Label

This PR also adds the missing "logo" property for a CarrierAccount.

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.

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)