admin api: implement PreviewClusterLayoutChanges

doc/api/garage-admin-v2.json

@@ -950,6 +950,30 @@
         }
       }
     },
+    "/v2/PreviewClusterLayoutChanges": {
+      "post": {
+        "tags": [
+          "Cluster layout"
+        ],
+        "description": "\nComputes a new layout taking into account the staged parameters, and returns it with detailed statistics. The new layout is not applied in the cluster.\n\n*Note: do not try to parse the `message` field of the response, it is given as an array of string specifically because its format is not stable.*\n    ",
+        "operationId": "PreviewClusterLayoutChanges",
+        "responses": {
+          "200": {
+            "description": "Information about the new layout",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/PreviewClusterLayoutChangesResponse"
+                }
+              }
+            }
+          },
+          "500": {
+            "description": "Internal server error"
+          }
+        }
+      }
+    },
     "/v2/PurgeBlocks": {
       "post": {
         "tags": [
@@ -2992,6 +3016,39 @@
           }
         }
       },
+      "PreviewClusterLayoutChangesResponse": {
+        "oneOf": [
+          {
+            "type": "object",
+            "required": [
+              "error"
+            ],
+            "properties": {
+              "error": {
+                "type": "string"
+              }
+            }
+          },
+          {
+            "type": "object",
+            "required": [
+              "message",
+              "newLayout"
+            ],
+            "properties": {
+              "message": {
+                "type": "array",
+                "items": {
+                  "type": "string"
+                }
+              },
+              "newLayout": {
+                "$ref": "#/components/schemas/GetClusterLayoutResponse"
+              }
+            }
+          }
+        ]
+      },
       "RemoveBucketAliasRequest": {
         "allOf": [
           {