feat: support DROP USER

doc/sql/query.ebnf

sbroad-cartridge/src/api/exec_query.rs

sbroad-cartridge/test_app/test/integration/acl_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/ir.rs

sbroad-core/src/frontend/sql.rs

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

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

sbroad-core/src/ir.rs

sbroad-core/src/ir/acl.rs

sbroad-core/src/ir/api/parameter.rs

sbroad-core/src/ir/distribution.rs

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

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

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

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