dotnet trace --format speedscope does not contain an All Threads item

## Background and Motivation

I've been using dotnet trace with speedscope, and you can only look at one thread at a time. It would be very useful to look at all the threads at once.

## Proposed Feature

Add an entry in the speedscope report format that contains the aggregated information of all the threads.

## Usage Examples

No change in user interaction with dotnet-trace itself.