feat: implement count, sum

sbroad-cartridge/test_app/test/integration/groupby_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/ast.rs

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

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

sbroad-core/src/ir.rs

sbroad-core/src/ir/aggregates.rs

sbroad-core/src/ir/explain.rs

sbroad-core/src/ir/expression.rs

sbroad-core/src/ir/function.rs

sbroad-core/src/ir/operator.rs

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