feat(plugins): add data migration mechanism for plugins

src/governor/plan.rs

src/luamod.rs

src/plugin/migration.rs

src/plugin/mod.rs

src/schema.rs

src/sql.rs

test/conftest.py

test/int/test_basics.py

test/int/test_plugin.py

test/testplug/testplug_w_migration/author.db

test/testplug/testplug_w_migration/book.db

test/testplug/testplug_w_migration/manifest.yaml