feat: support CASE expression

doc/sql/query.ebnf

.../test_app/test/integration/arbitrary_expressions_test.lua

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

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

sbroad-core/src/executor/ir.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/query.pest

sbroad-core/src/ir.rs

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

sbroad-core/src/ir/explain.rs

sbroad-core/src/ir/expression.rs

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

sbroad-core/src/ir/helpers.rs

sbroad-core/src/ir/transformation.rs

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

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

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

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