Add JSON report output (`--out-json`)

[sokovmd]

Summary

Add support for exporting benchmark results to a JSON file.

Today pgcompare generates an HTML report, which is great for visual analysis.
For automation (CI, scripts, comparisons across runs, dashboards), we also need structured JSON output.

Motivation

• simplify automated analysis in CI/CD
• enable programmatic comparison between runs
• allow long-term storage of results for trend analysis

Proposed UX

Add a new flag:

pgcompare run --config ./pgcompare.yaml --out ./report.html --out-json ./report.json

Behavior:

• --out-json is optional
• if omitted, current behavior is unchanged
• if provided, write full benchmark/report data to JSON in addition to HTML