feat: support union

doc/sql/query.ebnf

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

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

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

sbroad-core/src/backend/sql/tree.rs

sbroad-core/src/executor.rs

sbroad-core/src/executor/bucket.rs

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

sbroad-core/src/executor/ir.rs

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

sbroad-core/src/executor/vtable.rs

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

sbroad-core/src/frontend/sql.rs

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

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

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

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

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

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

sbroad-core/src/ir.rs