OAPI: add operation to fetch all workers

This introduces a new tag `worker-mgt` for worker management operations.
These are used by the web interface to display & manage workers.

pkg/api/flamenco-openapi.yaml

@@ -249,6 +249,20 @@ paths:
               schema:
                 $ref: "#/components/schemas/Error"
 
+  # Worker Management
+  /api/worker-mgt/workers:
+    summary: Obtain list of Workers known to the Manager.
+    get:
+      operationId: fetchWorkers
+      summary: Get list of workers.
+      tags: [worker-mgt]
+      responses:
+        "200":
+          description: Known workers
+          content:
+            application/json:
+              schema: { $ref: "#/components/schemas/WorkerList" }
+
   ## Jobs
 
   /api/jobs/types:
@@ -656,6 +670,8 @@ tags:
     description: Job & task queries, submission, and management.
   - name: worker
     description: API for Flamenco Workers to communicate with Flamenco Manager.
+  - name: worker-mgt
+    description: Worker Management API, for the web interface to query and control Workers.
   - name: shaman
     description: Shaman API, for file uploading & creating job checkouts.
 
@@ -1341,6 +1357,36 @@ components:
       enum: [job, tasklog]
       description: What kind of thing to subscribe to / unsubscribe from.
 
+    # Worker Management
+
+    WorkerList:
+      type: object
+      description: List of workers.
+      properties:
+        "workers":
+          type: array
+          items: { $ref: "#/components/schemas/WorkerSummary" }
+      required: [workers]
+
+    WorkerSummary:
+      type: object
+      description: Basic information about a Worker.
+      properties:
+        "id": { type: string, format: uuid }
+        "nickname": { type: string }
+        "status": { $ref: "#/components/schemas/WorkerStatus" }
+        "status_requested": { $ref: "#/components/schemas/WorkerStatus" }
+        # These will be implemented soon:
+        # "task_id":
+        #   type: string
+        #   format: uuid
+        #   description: The worker's current/last-worked-on task.
+        # "last_seen":
+        #   type: string
+        #   format: date-time
+        #   description: Last time this worker was seen by the Manager.
+      required: [id, nickname, status]
+
   securitySchemes:
     worker_auth:
       description: Username is the worker ID, password is the secret given at worker registration.