Add confidential filter for board list issues

What does this MR do and why?

Related to #337043 (closed)

In this MR we add a new :confidential argument to BoardIssuableInputBaseType and BoardIssueInputBaseType, to have the ability to filter issue boards by confidentiality in GraphQL.

Example queries

boardLists

query boardLists {
  project(fullPath: "gitlab-org/gitlab-test") {
    board(id: "gid://gitlab/Board/1") {
      lists(issueFilters: { confidential: true }) {
      	edges {
          node {
            issuesCount
          }
        }
      }
    }
  }
}

boardListIssues

query boardListIssues {
  project(fullPath: "gitlab-org/gitlab-test") {
    board(id: "gid://gitlab/Board/1") {
      lists(first: 1){
        edges {
          node {
            issues(filters: {confidential: true}) {
              edges {
                node {
                  title
                }
              }
            }
          }
        }
      }
    }
  }
}

epicBoardLists

query epicBoardLists {
  group(fullPath: "gitlab-org") {
    epicBoard(id: "gid://gitlab/Boards::EpicBoard/1") {
      lists(epicFilters: { confidential: true }) {
        edges {
          node {
            epicsCount
          }
        }
      }
    }
  }
}

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