feat: add simple on conflict clause for insert

sbroad-cartridge/test_app/test/data/config.yml

sbroad-cartridge/test_app/test/integration/insert_test.lua

sbroad-core/src/backend/sql/ir/tests.rs

sbroad-core/src/errors.rs

sbroad-core/src/executor/engine/helpers.rs

sbroad-core/src/frontend/sql.rs

sbroad-core/src/frontend/sql/ast.rs

sbroad-core/src/frontend/sql/ir/tests.rs

sbroad-core/src/frontend/sql/ir/tests/single.rs

sbroad-core/src/frontend/sql/query.pest

sbroad-core/src/ir/explain.rs

sbroad-core/src/ir/explain/tests.rs

sbroad-core/src/ir/operator.rs

sbroad-core/src/ir/operator/tests.rs

sbroad-core/src/ir/transformation/redistribution/insert.rs