Fix validate custom_field on JSON value instead of label

[menselman]

Terraform validate currently only allows the labelJSON on custom_fields, which is 'JSON'. See example below:

$ terraform validate
│ Error: expected type to be one of [text integer boolean date url select multiselect JSON], got json
│ 
│   with netbox_custom_field

But the actual netbox API when deploying a custom_field is expecting the valueJSON 'json'. See example below:

│ Error: [POST /extras/custom-fields/][400] extras_custom-fields_create default  map[type:[JSON is not a valid choice.]]
│ 
│   with module.am_netbox_tenants.netbox_custom_field,

Looking also at the other validators on L36-L42, this seems like a typo/mistake

[fbreckle]

I added a test that verifies the wrong behavior, i.e. the test fails without your fix but succeeds with it.