Move bundle download to struct API and bullet stream

[schneems]

Details in commits. The handle layer caching logic is exactly the same as the Ruby install layer

buildpacks-ruby/buildpacks/ruby/src/layers/ruby_install_layer.rs

Lines 40 to 58 in ec57995

	let layer_ref = cached_layer_write_metadata(layer_name!("ruby"), context, metadata)?;
	match &layer_ref.state {
	LayerState::Restored { cause } => {
	bullet = bullet.sub_bullet(cause);
	}
	LayerState::Empty { cause } => {
	match cause {
	EmptyLayerCause::NewlyCreated => {}
	EmptyLayerCause::InvalidMetadataAction { cause }
	| EmptyLayerCause::RestoredLayerAction { cause } => {
	bullet = bullet.sub_bullet(cause);
	}
	}
	let timer = bullet.start_timer("Installing");
	install_ruby(metadata, &layer_ref.path())?;
	bullet = timer.done();
	}
	}
	Ok((bullet, layer_ref.read_env()?))