Add a macro for complexes multiparts

[sobrinho]

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.

[sobrinho]

@rswag any news on that?

[oblakeerickson]

@sobrinho I'll take a look at it this week. I agree it is totally busy work to specify all the the individual let statements, so if we could get rid of that, it would be super helpful.

[oblakeerickson]

@sobrinho I ended up reverting this merge due to some tests that were failing. Please see #407.

I'm happy to get this merged in once we figure out what is going on with those failing tests. I probably should have merged the latest master into your branch to check for failures first before merging, sorry about this. I think you will have to open up a new pr when you are ready.