Support custom field filtering on /company and /contact endpoints

Description:

The filters parameter on GET /public-api/company and GET /public-api/contact currently accepts only a whitelisted set of fields. Custom fields (e.g. KvK-nummer) are rejected with a whitelist validation error. Customers with custom fields on these entities have no way to filter by them via the API and must fetch all records and search client-side.

Requested by W.T. Solutions (Peter van der Waal) for both company and contact endpoints.

Per dev confirmation this is something that can be opened up.

Acceptance criteria:

Custom fields are filterable via the filters param on both /company and /contact. Behavior should be consistent with how other filter types work (exact match on the custom field value).