[trace] Scaffold "thread trace dump instructions"

Depends on D88841

As per the discussion in the RFC, we'll implement both

  thread trace dump [instructions | functions]

This is the first step in implementing the "instructions" dumping command.

It includes:

- A minimal ProcessTrace plugin for representing processes from a trace file. I noticed that it was a required step to mimic how core-based processes are initialized, e.g. ProcessElfCore and ProcessMinidump. I haven't had the need to create ThreadTrace yet, though. So far HistoryThread seems good enough.
- The command handling itself in CommandObjectThread, which outputs a placeholder text instead of the actual instructions. I'll do that part in the next diff.
- Tests

{F13132325}

Differential Revision: https://reviews.llvm.org/D88769