Allow custom payloads for Profiler Frame and Scopes

[Wumpf]

See also #49

Making the profiler generic over two payload types doesn't seem to outrageous (with both defaulting to ()). Need to figure out how to keep things ergonomic though for users that don't want to supply payloads to their scopes & frames!

[Imberflur]

A more minimal approach that would address the cases in #49 could be to add a FrameId that is returned from GpuProfiler::end_frame and GpuProfiler::process_finished_frame. This could avoid the generics and ergonomics concerns and would be easier to implement.

For now I discovered I can probably pass an identifier through via the label for the top level query that covers the frame, but this isn't the cleanest approach.