Allow custom payloads for Profiler Frame and Scopes

[Wumpf]

See also https://github.com/Wumpf/wgpu-profiler/discussions/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!