Change rack_type schema to use lists rather than maps

[chrismarget-j]

The following attributes of a rack_type each use maps keyed by attribute instance name:

• leaf switch
• access switch
• generic system
• access switch links
• generic system links

This is an inelegant solution to two problems:

1. a since-resolved terraform plugin framework issue with nested sets
2. a misunderstanding that apstra doesn't preserve order for these objects

With both of these problems behind us it looks like these attributes can be either sets or lists.

Sets are possible/safe because each attribute has a name field which is required by Apstra to be unique.

Lists are possible because (we now believe) that Apstra preserves the order in which the object were submitted.

With that in mind, lists appear to be the best approach.