feat: add `explain` to the grammer and engine

benches/engine.rs

cartridge/roles/sbroad-router.lua

src/api.rs

src/api/explain.rs

src/executor.rs

src/executor/engine.rs

src/executor/engine/cartridge/router.rs

src/executor/engine/mock.rs

src/executor/tests/frontend.rs

src/frontend/sql.rs

src/frontend/sql/ast.rs

src/frontend/sql/ir.rs

src/frontend/sql/query.pest

src/ir.rs

src/router.lua

test_app/test/integration/api_test.lua

tests/artifactory/backend/sql/tree/sql_order_selection.yaml

tests/artifactory/ir/distribution/join_unite_keys.yaml

tests/artifactory/ir/distribution/shrink_dist_key_1.yaml

tests/artifactory/ir/distribution/shrink_dist_key_2.yaml