Add CpuProfileDataJson extension type for handling this data (#6980)

An alternative to some of the code in https://github.com/flutter/devtools/pull/6948

Work towards https://github.com/flutter/devtools/issues/6929

This one is again fairly simple. `CpuProfileData.parse` used to handle all of the parsing as statements, and relies on a fair amount of dynamic calling. When we extract the field calls into extension type getters, the casting etc is isolated to that section of the code. The statements in `CpuProfileData.parse` get simpler.

This could probably be improved more; I think `CpuProfileMetaData()` should just accept a `_CpuProfileDataJson` and get the data it needs. But that can be for later. Or now. Also all of the null-aware code could also be moved into the extension type. Depending on the business logic. Is it ever important to know if `sampleCount` is `null`? Or can we default it to `0` at the root? Etc.