Add Mobile Vitals to Unity transactions

[bruno-garcia]

See: https://blog.sentry.io/2021/08/23/mobile-vitals-four-metrics-every-mobile-developer-should-care-about

This exists in iOS/Android/Flutter and React Native

We get some measurements in the Sentry Defenses game: https://github.com/getsentry/sentry-defenses/blob/ea264f2aadb13439db1cbd083c5a49160f11d6a6/game/Assets/Scripts/Game/GameStateFighting.cs#L57

Landing page of performance for Unity includes these measurements which today are empty