Add support for field_options 

Add the ability to add field_options (date format) to all Signature Request methods.

Should be able to do either:
```
request.FieldOptions = new FieldOptions
{date_format = "DATE FORMAT HERE"};
```

OR use the AdditionalParameters.Add()
```
client.AdditionalParameters.Add("field_options", "{\"date_format\": \"MM - DD - YYYY\"}");
```

(Internal reference number: 10744067)