Add snapshotCreatedOn to Resource response types in REST API

[keithmanville]

The response schemas for Resources do not include the snapshot's created on datetime. This field was overlooked because for most endpoints, the latest snapshot is returned, so the snapshot created on datetime is the same as the last modified on datetime. However, when browsing the history, the snapshot created on datetime is important.

Definition of done

• add a snapshotCreatedOn datetime field to the response schema to all Resource response schemas
• update the "build" functions to return add the field to the responses
• update tests to validate the new response schema
• merge the feature into dev