Added custom-field-choice-sets to nb_lookup.py

[MrDiba]

Related Issue

#1185

New Behavior

Since NetBox v3.6.0 there is an breaking change where the custom-fields and choices are split.

Contrast to Current Behavior

Currently there is no way to access the custom field choices when using NetBox v3.6.0 or higher

Discussion: Benefits and Drawbacks

Why do you think this project and the community will benefit from your proposed change?

Starting from NetBox v3.6.0 it is not possible to use the lookup module to get the custom field choices.

What are the drawbacks of this change?

I do not know what happens when someone using it with an NetBox version lower than v3.6.0.

Is it backwards-compatible?

NetBox <v3.6.0; Use custom-fields to get the choices. Netbox >v3.6.0; Use custom-field-choice-sets

Changes to the Documentation

...

Proposed Release Note Entry

Added custom-field-choice-sets to the lookup plugin.

Double Check

• [x] I have read the comments and followed the CONTRIBUTING.md.
• [x] I have explained my PR according to the information in the comments or in a linked issue.
• [x] My PR targets the devel branch.