Review our use of the `default` OpenAPI spec feature

[j616]

Context
The TAMS spec has avoided the use of OpenAPI spec's default feature as it can lead to unexpected behaviour in some cases. For example, libraries that validate requests against the spec sometimes automatically set the defaults. We should review how widely we use the default feature and provide clear guidance on its use.

Actions

• Review our use of the OpenAPI spec default feature in TAMS, including where it may result in unexpected behaviour
• Review the impact the change will have on implementations
• Write up recommendations for its use in TAMS
• Roll out these recommendations across its spec

Outcome

• Application Note on use of the OpenAPI spec default feature
• Spec changes as appropriate