pageserver: add generation fields in openapi spec (#5690)

These optional fields have existed for as while, but weren't mentioned
in `openapi_spec.yaml` yet.

pageserver/src/http/openapi_spec.yml

@@ -713,6 +713,12 @@ paths:
 
         Errors if the tenant is absent on disk, already present in memory or fails to schedule its load.
         Scheduling a load does not mean that the tenant would load successfully, check tenant status to ensure load correctness.
+      requestBody:
+        required: false
+        content:
+          application/json:
+            schema:
+              $ref: "#/components/schemas/TenantLoadRequest"
       responses:
         "202":
           description: Tenant scheduled to load successfully
@@ -1203,6 +1209,15 @@ components:
             new_tenant_id:
               type: string
               format: hex
+            generation:
+              type: integer
+              description: Attachment generation number.
+    TenantLoadRequest:
+      type: object
+      properties:
+        generation:
+          type: integer
+          description: Attachment generation number.
     TenantAttachRequest:
       type: object
       required: