Proposed core fields

We should indicate which fields are core fields so we can build DD compliance testing into our own tools (`retsly-import vendor test`, etc).

```js
{
  OfficeAssociationComments: {type:String, maxlength:500, core:false},
  OfficeStatus: {type:String, maxlength:25, core:true},
  /* ... */
}
```

Attached:

- [RESO Data Dictionary 1.3 Core Fields v03.xlsx](https://files.zenhub.io/5421bb318c56be24496d93b9)