Fix generator to allow unknown fields in GO clients

[rjtokenring]

Motivation

Fix generator to allow unknown fields in GO clients.
Adding disallowAdditionalPropertiesIfNotPresent to False
https://openapi-generator.tech/docs/generators/go

Reviewer checklist

• PR address a single concern.
• PR title and description are properly filled.
• Changes will be merged in master.
• Changes are covered by tests.
• Logging is meaningful in case of troubleshooting.
• History is clean, commit messages are meaningful (see CONTRIBUTING.md) and are well formatted.
• Mentioned the Analytics team when there are schema changes.