[Feat] r.stats should have CSV output

[petrasovaa]

#3884 adds json format, but we should also add CSV format because it's a natural format for r.stats output. The CSV file should include header.

Optionally, we could also add JSONL format that could be advantageous for reading e.g. in pandas. The current JSON has a lot of hierarchy, the JSONL would be more similar to CSV, but with JSON advantages.