[FR]: Performance: Allow adding custom attributes to built-in traces

### Description

Firebase performance is tracking the number of built-in events, such as
- App lifecycle (`_app_start`,` _app_in_background`, `_app_in_foreground`)
- Network requests
- Time spent on a screen

Currently, it is not possible to assign custom attributes to those traces. It would be great to allow this. Luckily, it is a mere question of making an API public.

### API Proposal

`Performance` class has private compliance to `PerformanceAttributable` (FRPerformanceAttributable). If this compliance would be public, it would be possible to set the attributes on the `Performance` singleton, which are in turn propagated as default attributes for all traces.

### Firebase Product(s)

Performance