[Codegen] Convert "any type" to oneOf model

Created by: sebastien-rosset

Issue 1: In the JSON schema and OAS specifications, the array "items" attribute is NOT required. However I found out some language generators assume incorrectly that "items" is required. I am fixing the issue for:

• go-experimental
• python-experimental

https://json-schema.org/understanding-json-schema/reference/array.html

Issue 2 An OAS document may have a schema that does not specify the "type" attribute. For example, below notice the "type" attribute has not been set in the "annotation" property.

Pet:
  type: object
  properties:
    name:
      type: string
    annotations:
      description: an arbitrary element that can represent any JSON schema,
         The value may be null, integer, boolean, string, number, object, or array.

This is equivalent to writing:

anyOf:
  - type: 'null'
  - type: integer
  - type: string
  - type: number
  - type: object
  - type: array

To help with the code generation, I am proposing that codegen converts the "any type" to a anyOf composed schema. The alternative is every language generator has its own way to handle "any type", but it seems more generic to do this conversion. Potentially this could be controlled by a codegen runtime property.

PR checklist

• Read the contribution guidelines.
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