Establish a base AvailableObjectsView API view class

[jeremystretch]

Proposed Changes

Introduce a base API view class to serve the following endpoints:

• /api/asn-ranges/<id>/available-asns/
• /api/ip-ranges/<id>/available-ips/
• /api/prefixes/<id>/available-prefixes/
• /api/prefixes/<id>/available-ips/
• /api/vlan-groups/<id>/available-vlans/

Justification

Reduce boilerplate code and ensure consistent functionality across all related endpoints.