RFC: FastStore Feature Flag / Experimentation

[batzlerg]

Describe the feature you'd like
Support for injecting feature flag / experiment values at the top level of the app, to be consumed anywhere.

Proposed implementation is a top-level context + provider, populated by a user-defined function to provide experiment values which implements a specified interface. The provider can then wrap the core Component in _app.tsx, like the current UIProvider implementation.

Describe alternatives you've considered
A GraphQL API extension which allows users to query experiment values along with data. Rejected internally due to lack of flexibility in use cases such as running experiments inside API resolvers, preferred

Additional context
An RFC doc with more specific implementation ideas will be provided to the VTEX FastStore Experience team.