fix(rules_download): make intergrity optional

This change makes it so that if the user does not know the
intergrity when adding a new download, the framework will
notify them with the correct intergrity on the first download.

This is a better DevEx as otherwise the user needs to manually
download and checksum the file first.

This approach will also fail more gracefully when using mirrors
that have dynamically generated files and cannot be easily pinned.