Add the ability to filter issue boards by confidentiality in GraphQL

To get completeness of add new filtered search to issue/epic boards, we need to keep adding tokens.

We need to add confidentiality filter to issue boards and epic boards

backend

We need to add confidential to be a filter on Board.lists(issueFilters: BoardIssueInput) and BoardList.issues(filters: BoardIssueInput)

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Feature Flag

We're working on refactoring filtered search on boards to use GraphQL. This issue is part of that work, and is behind the :issue_boards_filtered_search feature flag.