[bug] OpenAPI 3: schema items - Value MUST be an object and not an array

[zaakn]

Related to #83, #84

After the PR was merged, it was rolled back in this commit:
540b249#diff-38bb6d02d3442ac95d8613ef4fed0b1cb70f7bc2001d1abe71bafbede04a0dd1

 - SchemaOrReference items = 30;
 + ItemsItem items = 30;

As the comments in the latest code say, I also agree that the wrong model is being used.

gnostic/surface/model_openapiv3.go

Lines 386 to 397 in ad271d5

	case "array":
	// I do believe there is a bug inside of OpenAPIv3.proto. I think the type of "schema.Items.SchemaOrReference"
	// shouldn't be an array of pointers but rather a single pointer.
	// According to: https://swagger.io/specification/#schemaObject
	// The 'items' "Value MUST be an object and not an array" and "Inline or referenced schema MUST be of a Schema Object"
	for _, schemaOrRef := range schema.Items.SchemaOrReference {
	arrayFieldInfo := b.buildFromSchemaOrReference(name, schemaOrRef)
	if arrayFieldInfo != nil {
	fInfo.fieldKind, fInfo.fieldType, fInfo.fieldFormat, fInfo.enumValues = FieldKind_ARRAY, arrayFieldInfo.fieldType, arrayFieldInfo.fieldFormat, arrayFieldInfo.enumValues
	return fInfo
	}
	}

Using the protoc-gen-openapi:

message Foobar {
	repeated string domains = 1 [
		(openapi.v3.property).items.schema_or_reference = {
			schema: {
				format: "hostname";
			}
		}
	];
}

output OpenAPI yaml:

Foobar:
    type: object
    properties:
        domains:
            type: array
            items:
                - type: string
                - format: hostname

Expected:

Foobar:
    type: object
    properties:
        domains:
            type: array
            items:
                type: string
                format: hostname