system_tables.md: update system tables of plugins
Summary
- dump_spec.lua: add _pico_plugin_migration
- system_tables.md: update system tables of plugins
Address changes from picodata@9a3b2be5
picodata 24.5.0-210-g9a3b2be5
- Close #298 (closed), #299 (closed)
- Cherry-pick to: none
- Staging: