Refactor PerformanceTracer buffer type to use output event format (#48310)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/48310

Refactors the internal storage format of trace events buffered by `PerformanceTracer`.

Aligning with the emitted [Trace Event Format](https://docs.google.com/document/d/1CvAClvFfyA5R-PhYUmn5OOQtYMH4h6I0nSsKchNAySU/preview?pli=1&tab=t.0#heading=h.yr4qxyxotyw) enables us to simplify away the issue of defining and converting from any intermediate formats. This becomes desirable as we generalise to more event types and forthcoming browser-emulating `__metadata` events.

Changelog: [Internal]

Reviewed By: hoxyq

Differential Revision: D67337442

fbshipit-source-id: 580928dfb4fcf4ac5efc82f93ab85a0d1d6dfb5c