feat: add replicate plugin for external data sources

[akamabu]

Summary

• Adds a new ReplicatePlugin that pulls data from an external StarbaseDB data source into the internal Durable Object SQLite database
• Supports explicit table configuration, cursor-based incremental replication, and table discovery
• Adds /replicate/run and /replicate/status admin endpoints
• Exports the plugin from @outerbase/starbasedb/plugins
• Adds Vitest coverage for plugin construction and error paths

Closes #72

Test Plan

• npx vitest run plugins/replicate/index.test.ts