Add custom SQL query support and proxy configuration

[irjudson]

Summary

Implements two new features for enhanced BigQuery sync capabilities:

1. Custom SQL Query Support (Issue #24)

• Add optional customQuery field to table configuration
• Support @lastTimestamp variable substitution for incremental sync
• Automatically wrap custom queries with partitioning logic (MOD-based)
• Enable JOINs, transformations, and complex WHERE clauses
• Maintain distributed workload partitioning across cluster

Example:

- id: enriched_events
  customQuery: |
    SELECT 
      e.event_time as timestamp,
      e.vessel_id,
      v.vessel_name
    FROM \`maritime_tracking.events\` e
    LEFT JOIN \`maritime_tracking.vessels\` v ON e.vessel_id = v.id
    WHERE e.event_time > TIMESTAMP(@lastTimestamp)
  timestampColumn: timestamp
  targetTable: EnrichedEvents

2. Proxy Configuration (Issue #23)

• Add optional proxy configuration at bigquery level
• Support HTTP/HTTPS proxies with authentication
• Configure via proxy.enabled and proxy.url fields
• Plugin-level configuration (does not affect Harper replication)

Example:

bigquery:
  proxy:
    enabled: true
    url: "http://proxy.vz.com:8080"

Additional Changes

This PR also includes:

• bq-benchmark tool for measuring BigQuery to Harper throughput
• Updated README documentation
• 18 new tests for custom query and proxy features

Testing

• ✅ All 91 existing tests pass
• ✅ All 18 new tests pass
• ✅ Linter passed
• ✅ Pre-commit hooks passed

Closes

Closes #23, Closes #24