Add support for multiple OpenAPI documents

[skryukov]

This PR adds support for multiple OpenAPI documents:

# rails_helper.rb
RSpec.configure do |config|
  # You can use different RSpec filters if you want to test different API descriptions.
  # Check RSpec's config.define_derived_metadata for better UX.
  config.include Skooma::RSpec[bar_openapi, path_prefix: "/bar"], :bar_api
  config.include Skooma::RSpec[baz_openapi, path_prefix: "/baz"], :baz_api
end

# spec/requests/bar/bar_spec.rb
describe "Bar API", :bar_api, type: :request do
  describe "GET /bar" do
    subject { get "/bar" }

    it { is_expected.to conform_schema(200) }
  end
end

To avoid having to specify the RSpec filter on every spec, you can use RSpec's config.define_derived_metadata:

# rails_helper.rb
RSpec.configure do |config|
  config.define_derived_metadata(file_path: %r{/spec/requests/bar}) do |metadata|
    metadata[:bar_api] = true
  end
end