DMP-71: Add list staffs API

docs/specs/71-add-list-staffs-api.md

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+---
+author: Prnyself <https://github.com/Prnyself>
+status: draft
+updated_at: 2021-05-19
+---
+
+# DMP-71: Add list staffs API
+
+## Background
+
+When creating a task, we should choose on which staff the task run. For now, we have already inserted staffID into DB,
+so we should support an `listStaffs` API for frontend to list all available staffs.
+
+## Propose
+
+So, I propose to design the list staff API to be called by the front-end, whose definition is as follows:
+
+```graphql
+type Query {
+    staffs: [Staff!]!
+}
+
+type Staff {
+    id: String!
+}
+```
+
+For now, we only record staff ID, and more information will be added in the future.
+
+In this way, when `createTask` is called, the `staffID` could be filled by the result of `listStaffs`.
+
+## Rationale
+
+The `createTask` input is defined as follows:
+
+```graphql
+input CreateTask {
+    name: String!
+    type: TaskType!
+    storages: [StorageInput!]!
+    options: [PairInput!]!
+    staffs: [StaffInput!]!
+}
+
+input StaffInput {
+    id: String!
+}
+```
+
+## Compatibility
+
+None
+
+## Implementation
+
+Most of the work would be done by the author of this proposal.