Add custom_fields to the vlan datasource

[a-willem]

Netbox has build-in customfields management, while it seems from PR #143 that general custom fields deployment (specifically within resources) seems complicated, adding them to the datasources is a minium enabling Netbox
users to pass their specific data onwards.

Tests seem to be passing :

--> make test
go test netbox/*.go -timeout=120s -parallel=4 -cover
go: downloading github.com/stretchr/testify v1.8.2
go: downloading github.com/davecgh/go-spew v1.1.1
go: downloading github.com/pmezard/go-difflib v1.0.0
ok command-line-arguments 0.365s coverage: 2.9% of statements

[fbreckle]

Hi! Thanks for your MR!
Can you please add a test that actually uses the new attribute?

[a-willem]

Hi ! Sorry for taking so long answering your request.

While I might be able to identify the right code and duplicate it to grab custom fields, I'm evidently not Go-fluent enough to write a unit-test requiring to setup the whole thing. If anyone'd like to help over here and/or take over the PR, feel free !