Add checksums

Some folks would like checksums alongside the `.vsix` files they can download to verify extensions. As far as I understand, the ask is only to upload checksums while adding extensions, and possibly to add them for existing extensions, nothing else.

Adding a checksum while adding an extension is easy, but not entirely sure about existing extensions. Have a command to crawl through and check extensions, or do it on demand when a checksum is requested, or ask users to re-add all their extensions if they need the checksums?