GraphQL: Expose issues under board lists

As part of #218040 (closed).

Query (group):

query {
  group(fullPath: "group") {
    board(id: "gid://gitlab/Board/3") {
      name
      lists {
        node {
          id
          issues {
            nodes {
              title
            }
          }
        }
      }
    }
  }
}

Query (project):

query {
  project(fullPath: "group/project") {
    board(id: "gid://gitlab/Board/3") {
      name
      lists {
        node {
          id
          issues {
            nodes {
              title
            }
          }
        }
      }
    }
  }
}