Allow filtering on executionState on WhereObservation

[hugo-vrijswijk]

It would be useful for navigate to filter only on ready or ongoing observations. This could be done client-side, but is probably more efficient server-side.

Proposal: add executionState to the WhereObservation param:

query observations {
  observations(
    WHERE: { executionState: { IN: ["NOT_STARTED", "COMPLETED"] } }
  ) {
    matches {
      id
      title
      execution {
        state
      }
    }
  }
}

[tpolecat]

Agree. This isn't straightforward right now because the status computation is complex. Working on it.