Add filtering and sorting to work item children

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Currently, we cannot filter or sort work item children. However, to implement the existing (with legacy epics) roadmap functionality, we will need the following set of variables applicable to work item children:

$state: IssuableState
$sort: WorkItemSort
$timeframe: Timeframe
$labelName: [String!]
$authorUsername: String = ""
$confidential: Boolean
$search: String = ""
types: [EPIC] // constant here as we only need epic children for roadmaps now