Add a publicName field in the response of the get user endpoint

The Confluence GET /wiki/rest/api/user endpoint returns a publicName field which is documented as "The public name or nickname of the user. Will always contain a value.".

However, the Jira GET /rest/api/3/user endpoint does not contain a publicName field. 

Both endpoints return a displayName field, but the usefulness of the data in this field is dependent on the user's privacy setting so in many cases, this field is less useful for apps than a publicName field.