feat: implement string concatenation

doc/sql/feature_taxonomy.md

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

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

sbroad-core/src/executor/tests.rs

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

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

sbroad-core/src/frontend/sql.rs

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

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

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

sbroad-core/src/ir/explain.rs

sbroad-core/src/ir/explain/tests.rs

sbroad-core/src/ir/explain/tests/concat.rs

sbroad-core/src/ir/expression.rs

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

sbroad-core/src/ir/tree.rs