Add a macro for complexes multiparts

This will allow to describe multipart in a short way, like JSON payload:

Before:

    put 'Creates a blog with thumbnail' do
      consumes 'multipart/form-data'
      parameter name: :title, in: :formData, type: :string, required: true
      parameter name: :content, in: :formData, type: :string, required: true
      parameter name: :file, in: :formData, type: :file, required: true

      let(:blog) { FactoryBot.build(:blog) }
      let(:title) { blog.title }
      let(:content) { blog.content }
      let(:file) { blog.file }

      ...
    end

After:

    put 'Creates a blog with thumbnail' do
      consumes 'multipart/form-data'
      parameter name: :blog, in: :formData, schema: { '$ref' => '#/definitions/blog' }

      let(:blog) { FactoryBot.attributes_for(:blog) }

      ...
    end

Your mileage may vary but you can always choose the best option.

test-app/swagger/v1/swagger.json

@@ -155,6 +155,52 @@
         }
       }
     },
+    "/blogs/multipart": {
+      "post": {
+        "summary": "Creates a blog using multipart",
+        "tags": [
+          "Blogs"
+        ],
+        "description": "Creates a new blog from provided data",
+        "operationId": "createBlogWithMultipart",
+        "parameters": [
+
+        ],
+        "responses": {
+          "201": {
+            "description": "blog created",
+            "content": {
+            }
+          },
+          "422": {
+            "description": "invalid request",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/definitions/errors_object"
+                }
+              }
+            }
+          }
+        },
+        "requestBody": {
+          "content": {
+            "multipart/form-data": {
+              "schema": {
+                "type": "object",
+                "properties": {
+                  "name": "blog",
+                  "type": "object",
+                  "properties": {
+                    "$ref": "#/definitions/blog"
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/blogs/flexible": {
       "post": {
         "summary": "Creates a blog flexible body",