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/spec/features/swagger_ui_spec.rb

@@ -7,6 +7,7 @@ RSpec.feature 'swagger-ui', js: true do
 
     expect(page).to have_content('GET /blogs Searches blogs', normalize_ws: true)
     expect(page).to have_content('POST /blogs Creates a blog', normalize_ws: true)
+    expect(page).to have_content('POST /blogs/multipart Creates a blog using multipart', normalize_ws: true)
     expect(page).to have_content('GET /blogs/{id} Retrieves a blog', normalize_ws: true)
   end
 end