feat: add support for NetBox custom fields

[fmcglinn]

Allow using custom fields (prefixed with cf_) as the source for DNS names. This allows multiple DNS records without hijacking the description field.

The field_name parameter now accepts:

• Standard fields: dns_name, description
• Custom fields: cf_<field_name>

Example Use Case

• Use the dns_name field for reverse DNS (PTR records)
• Use a custom field additional_dns for forward DNS (A/AAAA records) to allow multiple services.

providers:
  # Provider for reverse DNS - uses dns_name field
  # This creates PTR records pointing to the canonical hostname
  netbox:
    class: octodns_netbox.NetboxSource
    url: env/NETBOX_URL
    token: env/NETBOX_TOKEN
    ttl: 3600
    field_name: dns_name

  # Provider for forward DNS - uses custom field
  # This creates A/AAAA records for service aliases
  netbox-additional:
    class: octodns_netbox.NetboxSource
    url: env/NETBOX_URL
    token: env/NETBOX_TOKEN
    ttl: 3600
    field_name: cf_additional_dns

zones:
  example.com.:
    sources:
      - netbox    # dns_name field
      - netbox-additional    # cf_additional_dns
    targets:
      - output

  # Reverse zone - uses the dns_name provider
  2.0.192.in-addr.arpa.:
    sources:
      - netbox
    targets:
      - output