Automatic transactions when a scene loads

[bruno-garcia]

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If a tracing sampling rate is defined and higher than 0 (aka: Performance is enabled) we want to start transactions as a result to a scene being loaded.

docs.unity3d.com/ScriptReference/SceneManagement.SceneManager-sceneLoaded.html

If we can finish the transaction when scene finished loaded, we can use the waitForChildren option which exists in the iOS/Android/Flutter SDK and isn't documented yet on develop docs.

• [x] Create transaction when scene starts to load
• [x] Finish transaction on scene completes loading
• [ ] When all child spans finish, capture the transaction (timestamp is adjusted here)
• [ ] Document waitForChildren bahavior in the develop docs, and link to some example implementation