system_tables.md: add _pico_plugin_config

Summary

• dump_spec.lua: add _pico_plugin_config
• system_tables.md: add _pico_plugin_config

Address changes from picodata@97f4637f

picodata 24.5.0-231-g97f4637f

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• Follow-up for picodata!1157 (comment 95278)
• Close nothing
• Cherry-pick to: none
• Staging: https://docs.binary.picodata.io/picodata/asabirov_update_system_tables/architecture/system_tables/#_pico_plugin_config