Provide resource for finding available interface by name

[mraerino]

This resource is similar to netbox_available_ip_address as it allows creating a resource based on what other resources are present in a certain namespace.

In this case we're looking at all interfaces of a device with a certain prefix and create an interface with the lowest number suffix to a prefix so that it doesn't clash with an existing interface.

Notably, it doesn't use an existing API in Netbox to accomplish this, but instead does an API call to list the interfaces and constructs the name in the provider. This might be prone to race conditions when creating multiple interfaces for the same device with the same prefix at the same time.

[fbreckle]

This is absolutely an interesting feature, but I am not sure if implementing this kind of logic directly into the provider is a good pattern. Any thoughts on that?

I will postpone this until after 4.x compatibility.

[mraerino]

yeah, it's debatable - i mostly just really need this functionality and it's really hard to accomplish without support in the provider. i might try to get this supported in the netbox API