Remove use of interface{} in types in favor of a struct

[jessesuen]

Is this a BUG REPORT or FEATURE REQUEST?: FEATURE REQUEST

What happened:
In the API types, we have the following definition of Item

// Item expands a single workflow step into multiple parallel steps
type Item interface{}

The problem is that our use of the interface{} type does not play well with kube OpenAPI generator, nor with the kube DeepCopy generators. As a result, the withItems field is not included in the OpenAPI spec at the moment, and we have to override the DeepCopy function in the types. Instead of interface{}, we need to define a new type MapOrString similar in function to IntOrString with custom UnmarshalJSON feature.

Environment:

• Argo version: v2.0.0