feat: make it possible to validate configuration without the existence of a specific workspace

[GGabriele]

With https://github.com/Kong/deck/pull/502, we are going to introduce the capability to validate configuration against the Kong Admin API. We should improve such capability allowing users to validate their configuration without the existence of a specific workspace, before applying their changes.

Related discussion here: https://github.com/Kong/deck/pull/502#discussion_r732141388