Ensure mandatory parts of the spec are marked as required

[paulsturgess]

There are places in the spec where we can explicitly mark things as "required" and this has not been implemented everywhere.

For example in the Parameter object, required determines whether this parameter is mandatory.

• parameters: https://spec.openapis.org/oas/v3.0.0#fixed-fields-9
• request body: https://spec.openapis.org/oas/v3.0.0#fixed-fields-10
• at the schema level, we can define required properties: https://spec.openapis.org/oas/v3.0.0#models-with-composition