refcol: add JSON trace output mode to the ITT API reference collector

[eparshut]

Summary

This PR introduces an experimental capability to the ITT API reference collector: in addition to the existing plain-text log, it can now produce a JSON file that opens directly in Perfetto.
Developers who instrument their code with the ITT API can now visualize their traces with zero extra tooling: no profiler, no additional binaries, no conversion step. Point the reference collector at your app, set one environment variable, run, and drag the resulting trace file into Perfetto to see your tasks, regions, frames, counters, and metadata on a timeline.

Changes:

• New JSON output mode, opt-in via EXP_LIBITTNOTIFY_GEN_JSON=1. Default behavior (plain-text log) is unchanged.
• Trace event mapping for the core ITT primitives: tasks, regions, frames, counters, metadata with process/thread/timestamp attribution.
• Smoke test covering both plain text and JSON modes to guard the feature.
• Docs updated (README + reference docs).

How to use:

python buildall.py --refcol
export INTEL_LIBITTNOTIFY64=<build_dir>/libittnotify_refcol.so
export EXP_LIBITTNOTIFY_GEN_JSON=1
./run_itt_instrumented_app
# open the generated libittnotify_refcol_<timestamp>.json in https://ui.perfetto.dev: (screenshot below)