Change the internal representation of Workflow schemas in Designer

Up until now we've relied on a representation of a schema for a particular node (or task) that encapsulates and creates an abstraction for the variables schema (fields that are specific to the node) and the global variables schema (fields from the workflow that are made available on the node). This approach made sense because we never really needed to handle the fields differently, because in the context of a task layout they are accessed as if they belong to the same schema.

However, this oversimplification makes it harder to manipulate workflow fields when we need to distinguish the variables schema fields from the global variables schema fields. The goal is to maintain the separation of both schemas for each node, just as they are provided from the registries endpoint.