Hierarchy widget - extend work item create mutation with accepting parent/child widget

• this needs to be part of work item create mutation because we need to create parent/child relationship as part of work item creation action
• add a new graphql input type for passing parent ID reference (e.g. !85700 (diffs))
• add service-level layer for parent/child widget and hook it to work item create service (e.g. !85700 (diffs))
• ideally put widgets input under widgets: - !85700 (comment 957267576) (this is a change compared to the original draft)

Draft: !85700 (diffs)

SPIKE discussion: #358195 (comment 932136343)