Add an API endpoint to fetch entity information for IP devices

[lunkwill42]

Is your feature request related to a problem? Please describe.

The NetboxEntity model (netboxentity database table) is NAV's main way of storing information about physical entities discovered in IP devices (primarily through the ENTITY-MIB). This is, for example, the de-facto way of finding chassis devices in a switch stack (unless the device model in question models virtual chassis members as modules instead, like many Juniper devices do).

There is currently no endpoint to discover this entity metadata. We would like to get information about operational physical entities and their serial numbers for syncing to our inventory systems, but this is currently possible to do completely.

Describe the solution you'd like

A new read-only API endpoint netboxentity should be added, tied to the NetboxEntity model.