Add PerfettoTracing for detailed query execution profiling

[rmosolgo]

This extends GraphQL::Tracing to have some different hook methods and adds an implementation for the Perfetto Trace Viewer.

I was looking for something with better support for async trace rendering and discovered Perfetto. In the end, I'm not really using its parallel trace capability, but its custom "tracks" and "flows" are great because it can link field execution to Dataloader::Source#fetch calls.

TODO:

• Add new methods to base Trace class
• In another PR, update other tracing implementations to use begin/end hooks to support Fiber pauses better
• Add a development cop which checks for super in implementation methods
• Backwards compat for Dataloader#yield without args?
• Handle the optional dependency of google-protobuf nicely
• Test with AsyncDataloader
• Document