Gets custom fields
<br>
Returns the custom fields of the company named by the `company-id` header, active and
inactive alike; read `isActive` to tell them apart.
<br>
Requiredness is per service offering, so it is reported on each entry of
`serviceOfferings` rather than on the field as a whole. The same field can be required
for Contract and optional for Direct Hire.
<br>
`customFieldItems` is populated according to `customFieldTypeId`, and is only ever
null where there is nothing to fetch:
<list type="bullet">
<item>
<b>2, Custom List</b>: the field's selectable items, the same active set that
`GET /CustomFields/items` returns. No per-field call is needed.</item>
<item>
<b>1, Team Members</b>: null. The selectable values are the company's backend users,
the same set for every such field; fetch them once from
`GET /wf/v1/LookUps/TeamMembers`, which also carries the email address needed to
match a person.</item>
<item>
<b>3, Free Form</b>: null. The value is typed in, so there is nothing to select.</item>
</list>