Fetch latest "stable" build from update check endpoint

[FichteFoll]

Also support new "binary" value for packages ref to automatically use the same build tag as for the binary.

Addresses the second point of #6 that should arguably have been its own issue.

[deathaxe]

LGTM!

We may fetch tags from Packages repo to ensure to have a valid reference, even if binary points to not yet existing tag.

Maybe better to write an action for Packages which checks the update end point and creates tags automatically :-D

[FichteFoll]

Maybe better to write an action for Packages which checks the update end point and creates tags automatically :-D

The problem here is that we cannot simply tag the current master but need to check which version of the repo was actually included in the release. Anyway, that's for another time (and another repo).

[deathaxe]

LGTM!