Add EventPipe Processor Number support and make NetTrace the default … (#25276)

Add EventPipe Processor Number support and make NetTrace the default … (#25276)

* Add EventPipe Processor Number support and make NetTrace the default format

The EventPipe header now has a Processor Number field. On windows we query the correct value, on other OSes we currently have a -1 placeholder, but the field is written to the format regardless.
NetTrace is now the default format when using the environment variable, and the format must be explicitly configured when using the IPC channel or managed API. A parallel change in the diagnostics repo is changing dotnet-trace and dotnet-counter to specify nettrace format which means .NET devs should see nettrace almost exclusively from now on. If for whatever reason it is needed, NetPerf remains available if a scenario explicitly requests to use it.

* PR feedback + attempting to fix broken tests