dotnet: Merge stacks with per-process/per-thread identifiers

dotnet-trace produces stacks of this form: `Process64 Process(1) (1) Args: _[net];(Non-Activities)_[net];Threads_[net];Thread (1)`. The `Thread` and `Process` stacks have high cardinality - and prevent the merging of the same stacks across differnet machines, because different processes/threads will have different IDs across different machines.

If dotnet-trace supports removing them (like async-profiler supports embedding per-thread names, and supports not doing so) - let's use that flag and remove.
If not - then gProfiler (`dotnet.py`) should strip those parts away. IMO the frames can be removed altogether - the frames that create high cardinality, so `Process(xxx)` and `Thread(xxx)` frames.