Support cross slice assets

[timriley]

Update asset helpers to accept Hanami::Assets::Asset instances and use the URL from those, instead of trying to look up the asset from the assets in the view context.

See spec/integration/assets/cross_slice_assets_helpers_spec.rb (added in this PR) for a full demonstration, but in short, using assets from across slices involves:

(1) Importing the assets object from one slice to another:

module Admin
  class Slice < Hanami::Slice
    import keys: ["assets"], from: Hanami.app.container, as: :app
  end
end

(2) Making the imported assets component available in the view context:

module Admin
  module Views
    class Context < Hanami::View::Context
      include Deps[app_assets: "app.assets"]
    end
  end
end

(3) Using the asset objects directly with the assets helpers:

<%= javascript_tag(app_assets["app.js"]) %>

The recommended way to use assets is in full isolation, which is why I don't think we need to further streamline the steps above. But doing this shows how it is possible and should allow people to achieve cross-slice asset usage if their situation requires it.