Invite user to update when available

[Hoverbear]

Warning!!!!: This PR breaks the registry! latest_riff_version is a required field. We can make it optional if we desire.

This adds a latest_riff_version to the registry which we can then read later to let the user know a new version is available.

You should test this with --offline, and remove the $XDG_CACHE_DIR/fsm/registry.json (and edit the registry/registry.json) or edit the cached version.

Open questions

• [ ] There is no way to make this go away yet. Do we want something to hush this?
• [ ] We warn after we detect dependencies, should we warn before? Do we want the user's first output from the program to be an update message?

[linear[bot]]

DS-281 Include "latest version" information in the registry API to invite people to download an updated copy

[Hoverbear]

Gonna alter the behavior if we sent rubbish in this field from the registry update...