Create `upgrade_resource()` helper function

[peterdesmet]

This helper function should upgrade a v1 data resource to v2

• See the discussion issue How to convert a v1 to a v2 package #264 for the changes that need to be implemented.
• Write in upgrade_resource.R file
• Figure out when to call this function
  • Ideally not by read_package() as it might generate ERRORs for the user that blocks them from fixing it (since they can't read)
  • Maybe each time we call resource()? That would mean resources are only upgraded on e.g. write_package(), which is probably fine.