feature(plugin): add sql interface for plugins

sbroad

src/governor/plan.rs

src/luamod.rs

src/pgproto/backend/describe.rs

src/plugin/mod.rs

src/rpc/load_plugin_dry_run.rs

src/sql.rs

src/storage.rs

src/traft/node.rs

src/traft/op.rs

test/int/test_plugin.py

test/testplug/src/lib.rs

test/testplug/testplug/0.1.0/manifest.yaml

test/testplug/testplug/0.2.0/manifest.yaml

test/testplug/testplug_small_svc2/0.1.0/manifest.yaml

test/testplug/testplug_w_migration/0.1.0/manifest.yaml