[Tools] Allow cta-admin to print a separate JSON object per record

Description

This MR allow cta-admin stream commands to print one independent JSON object per output record (JSON Lines format), instead of putting all JSON objects inside the same JSON array.

It's enabled by adding a new --jsonl flag to commnd line arguments.

This enables many Linux tools, such as wc -l, to directly use the cta-admin --jsonl output.

Checklist

• Documentation reflects the changes made.
• Merge Request title is clear, concise, and suitable as a changelog entry. See our contributing docs

References

Closes #1324 (closed)