Improve integration between lib, UI, CMP UI, audit

[OlivierChirouze]

• "widget UI" and "CMP UI" rely on the PAF lib to interact with data (read, refresh and write cookies)
• the PAF lib needs to prompt user when they are unknown to PAF, and show a notification snackbar when relevant.

This creates a circular dependency that today is solved via the declaration of PAF and PAFUI global variables, that are re-declared in multiple parts of the app.

See https://github.com/prebid/paf-mvp-implementation/pull/120/files/5fb8a059d1d70fa226c13aba8c9655baad2ef549#diff-21193811b758f6d40b39e174d4b5622f8b741a520255c5351f18056bf93fe4c8R442

This should be improved.

We could reuse a pattern used by PrebidJS, by using a queue of actions with callback, for instance.