Support all of todos API in GraphQL

Problem

We can't implement the sidebar in GraphQL until this is done, ergo this is blocking "real time" sidebars.

Proposal

We should make it possible to do everything that https://docs.gitlab.com/ee/api/todos.html does using our GraphQL API.

We can use this issue to track the overall work; but propose creating multiple MRs that track the implementation parts outlined below.

ToDo Implementation Parts

• Query
• Mark as done
• Restore
• Bulk mark as done
• Bulk restore