Add 'createdAt' field support for creation in Mutation.workItemCreate API

Summary

It is not possible to set the createdAt value when creating Epic as WorkItem in Mutation.workItemCreate.

The Mutation.createIssue endpoint provides it:

createdAt: Timestamp when the issue was created. Available only for admins and project owners.

Work-around

Use the REST API:

• create epic (deprecated endpoint) provides the created_at attribute.
• create issue provides the created_at attribute.