feat: support grouping by expression

sbroad-cartridge/stress-test/groupby/init.lua

sbroad-cartridge/stress-test/groupby/k6.js

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

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

...ad-cartridge/test_app/test/integration/sql_cache_test.lua

sbroad-core/src/errors.rs

sbroad-core/src/executor/tests.rs

sbroad-core/src/executor/tests/subtree.rs

sbroad-core/src/frontend/sql.rs

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

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

sbroad-core/src/ir/expression.rs

sbroad-core/src/ir/operator.rs

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

sbroad-core/src/ir/tree/expression.rs