feature(plugins): add `migration_up` and `migration_down` functions (#close 683)

Summary

• feature(plugins): add migration_up and migration_down functions (#close 683)

• add migration_up and migration_down lua API functions
• now the migration mechanism in plugins takes into account the plugin version and, accordingly, previous applied migrations
• add _pico_plugin_migration system table

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• Closes #652 (closed), #683 (closed)
• Cherry-pick to: none
• Docs follow-up: docs#298 (closed) docs#299 (closed)