autorefresh: snapd refresh awareness ux implementation refresh available hook

[sergio-costas]

Implements an "user hook" called "refresh-available". This "user hook" is called as a user program, and must be defined in the snap's YAML file as an app entry with the name "userhook-refresh-available".

THIS IS STILL A DRAFT!!! This code is only a proof-of-concept to help discuss possible implementations for SD136.

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