Features/update announcements to match spec 178933309

[rosalinekarr]

Problem

We've updated the announcement spec since the SDK announcement module was written, so now the SDK is out of sync with the spec. Specifically, we're missing createdAt on all announcements. #178933309

Solution

Added createdAt to all announcement types and updated tests accordingly

Change summary:

• Added createdAt to all DSNP announcement types
• Added an optional createdAt arg to DSNP factories
• Updated tests accordingly
• Also fixed GraphChange enum out of sync with spec

Steps to Verify:

1. Call a create method from the announcements module
2. Check that the resulting object has a createdAt field

[rosalinekarr]

looks good to me except for the duplicate createdAt's that github found 👍

Fixed