Added `source` and `partner_name` to Company Benefits

Two new string fields source and partner_name are now available on the "Create a company benefit", "Get benefits for a company", "Get a company benefit", and "Update a company benefit" endpoints.

Company benefits created via the API default to "external." Certain partners can create company benefits with a source of "partnered." "Internal" company benefits are brokered by Gusto.

{
  "uuid": "d2cec746-caee-464a-bcaf-00d93f7049c9",
  "version": "98jr3289h3298hr9329gf9egskt3kagri32qqgiqe3872",
  "company_uuid": "881ce3f2-e3e1-49c9-8ad4-0bcf515f5618",
  "benefit_type": 1,
  "active": true,
  "description": "Kaiser Permanente",
  "source": "external",
  "partner_name": "XYZ Corp",
  "deletable": true,
  "supports_percentage_amounts": true,
  "responsible_for_employer_taxes": false,
  "responsible_for_employee_w2": false
}