Draft: Add CurrentUserTodos widget

What does this MR do and why?

Related issue: #393784 (closed)

Screenshots or screen recordings

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How to set up and validate locally

1. Feature.enable(:multiple_todos) if you want to test with multiple todos for a work item
2. Create an OKR, or any other work item (like Tasks) and get the global Work Item ID
3. Use the following GraphQL queries and mutations to test the feature:

query pendingTodos {
  workItem(id: "gid://gitlab/WorkItem/740") {
    id
    workItemType {
      id
      name
    }
    widgets {
      ... on WorkItemWidgetCurrentUserTodos {
        currentUserTodos(state:pending) {
          edges {
            node {
              id
              state
            }
          }
        }
      }
    }
  }
}

query doneTodos {
  workItem(id: "gid://gitlab/WorkItem/740") {
    id
    workItemType {
      id
      name
    }
    widgets {
      ... on WorkItemWidgetCurrentUserTodos {
        currentUserTodos(state:done) {
          edges {
            node {
              id
              state
            }
          }
        }
      }
    }
  }
}

mutation markAllTodosAsDone {
  workItemUpdate(input:{
    id:"gid://gitlab/WorkItem/740",
    currentUserTodosWidget:{
      action:MARK_AS_DONE
    }

  }) {
    clientMutationId
  }
}

mutation markTodoAsDone {
  workItemUpdate(input:{
    id:"gid://gitlab/WorkItem/740",
    currentUserTodosWidget:{
      todo_id: "gid://gitlab/Todo/40",
      action:MARK_AS_DONE
    }

  }) {
    clientMutationId
  }
}

mutation addNewTodo {
  workItemUpdate(input:{
    id:"gid://gitlab/WorkItem/740",
    currentUserTodosWidget:{
      action:ADD
    }

  }) {
    clientMutationId
  }
}

MR acceptance checklist

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