Add field selection and table formatting for parse/search commands

[digizeph]

Summary

This PR improves the output formatting for monocle parse and monocle search commands by adding:

• Field selection (--fields / -f): Choose which columns to display
• Table formatting: Proper bordered tables when using --format table
• Better defaults: PSV format for streaming, separate defaults for parse vs search

Changes

• New --fields option accepts comma-separated field names (e.g., -f prefix,as_path,collector)
• Table output now uses tabled crate with rounded borders instead of tab-separated values
• Default format changed to PSV (pipe-separated) for parse/search - works better for streaming large datasets
• Parse command defaults exclude the collector field (not applicable)
• Search command defaults include the collector field
• Markdown format includes proper header row with separator

Available Fields

type, timestamp, peer_ip, peer_asn, prefix, as_path, origin, next_hop, local_pref, med, communities, atomic, aggr_asn, aggr_ip, collector

Examples

# Default PSV output (no headers, streaming-friendly)
monocle search --start-ts 2025-01-01T00:00:00Z --duration 1m -c rrc00

# Table format with borders
monocle search --start-ts 2025-01-01T00:00:00Z --duration 1m -c rrc00 --format table

# Custom field selection
monocle search --start-ts 2025-01-01T00:00:00Z --duration 1m -c rrc00 -f prefix,as_path,collector

# Markdown output
monocle parse some-file.gz --format markdown

Closes #99