feat(settings): Add nimbus experiments to AppContext

[jonalmeida]

Because

• We want to fetch experiments that can target a user on the first render/launch.

This pull request

• Adds uniqueUserId generation which would happen in Backbone into React as a prerequiste.
• Adds a new call to experiments in the AppContext.

Issue that this pull request solves

Closes: FXA-9783

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

Other information (Optional)

• In development, useMemo is called twice because of StrictMode. This will not happen in a production build.
• Tests will be followed up in FXA-9784, but I'd happily take some links and pointers to equivalent features that I could base mine on top of.

[jonalmeida]

I want to get this into AppContext or some other kind of observable state so that I can write a very simple useExperiments which provides a consumer with the experiment info they need in that final form that they can use anywhere in the app.

I'm not sure what to do with the results from initializeNimbus here.

[jonalmeida]

Maybe, I should move this to AppContext.initializeAppContext?

[jonalmeida]

I think this is the right direction for what we want: the experiments are fetched before the first render.

I can see this happen very early on now:

[jonalmeida]

The latest patch now has my current path forward - it doesn't seem like the worst idea since we make gql fetches at the same place, it's going to be one more request that happens now.

[jonalmeida]

Placed this here for now, but I can move it to some other place.