Add setting to allow opt-out of Griffin variations

[iefremov]

Description

we should add a global toggle to preferences below the P3A preference, that would allow a user to opt-out of receiving new variations from griffin backend. This should not affect the variations master preference in #15229

Designs

This toggle should be located in brave://settings/privacy under the P3A option (for now).

[ShivanKaul]

Given that we're also planning to run user studies where we collect user data through Griffin (https://github.com/brave/security/issues/439), perhaps this user preference could read
Allow staged roll-outs, feature testing and user studies
This anonymized info helps Brave improve security and performance of current and future features.
?
We don't currently have any preferences related to user studies and ISTM that if the user studies we run are going to be based on Griffin, then this would be a good place to control that. It would be even better if we can let users see and opt-out of specific Griffin studies, kinda like Firefox allows via their about:studies.

[iefremov]

closing as mostly dupe of #36295 where the discussion actually happens