Relates to #12201

Code generated by the Go client generator does not validate required fields, which is especially problematic in oneOf schemas where the sub-schemas are differentiated by required fields.

For example, consider the following schema:

components:
  schemas:
    Thing:
      oneOf:
        - $ref: '#/components/schemas/ThingWithFoo'
        - $ref: '#/components/schemas/ThingWithBar'
    ThingWithFoo:
      type: object
      properties:
        foo:
          type: string
        id:
          type: string
    required:
      - foo
    ThingWithBar:
      type: object
      properties:
        bar:
          type: string
        id:
          type: string
    required:
      - bar

Assume that the API itself only returns an object that has foo or an object that has bar; it will not return an object with both properties. Existing versions of openapi-generator Go client generator will generate models that do not enforce required properties; as a result, any attempt to use the Thing model will result in an error like Error when calling <something>: data matches more than one schema in oneOf(Thing).

This PR replicates the required field validation that exists in the okhttp-gson generator:

• Models with required properties are generated with an array of required property names
• UnmarshalJSON is updated to unmarshal the JSON into a map with string keys
• If a required property is not found in the map, UnmarshalJSON returns an error

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  ./bin/utils/export_docs_generators.sh
  

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