Notify users when new version of Cog is available

[mattt]

Kudos to @fofr for suggesting this.

It would be helpful if Cog could tell users when a new version is available, similar to other tools (also on an opt-out basis).

For example, Replicate recently made a change to enforce the validity of OpenAPI schemas for model inputs and outputs. Releases prior to v0.9.5 had a problem (#1275) where models returning custom object would generate an invalid Output schema. A customer trying to push an existing model would see a new failure and may get stuck, despite the fix being available in a newer release.

As far as implementation, there are stronger and weaker forms this functionality could take. (Whatever we do, a hard requirement should be that users can easily opt out of automatic version checking).

• A light touch form of this would be for Cog to do a version check when the build or push subcommand fails.
• A slightly more obtrusive form would be to check as part of any cog command, possibly storing a timestamp for last check and only checking no sooner than the next day / week / etc.
• A more extreme form of this would be to require the latest version of Cog to run cog push to r8.im. This would be the most straightforward thing to implement, and has the advantage of being more like a required software upgrade than nagging with telemetry.

@replicate/cog Thoughts?