Support Clickhouse ASSUME table constraints (and remove required parenthetical)

Hello, currently [Clickhouse's ASSUME table constraints](https://clickhouse.com/docs/reference/statements/create/table#constraints) generate parse errors:
```bash
$ cat assume_test.sql
CREATE TABLE my_table (
`date` Date,
CONSTRAINT my_constraint ASSUME (date > '2025-01-01')
)
ENGINE = MergeTree
;
```
----
```bash
$ cargo run --features json_example --example cli assume_test.sql --clickhouse
Finished [`dev` profile [unoptimized + debuginfo]](https://doc.rust-lang.org/cargo/reference/profiles.html#default-profiles) target(s) in 0.01s
Running `target/debug/examples/cli assume_test.sql --clickhouse`
Parsing from file 'assume_test.sql' using ClickHouseDialect
2026-08-31T00:27:53.248Z DEBUG [sqlparser::parser] Parsing sql 'CREATE TABLE my_table (
`date` Date,
CONSTRAINT my_constraint ASSUME (date > '2025-01-01')
)
ENGINE = MergeTree
;
'...
Error during parsing: ParserError("Expected: PRIMARY, UNIQUE, FOREIGN, CHECK, or EXCLUDE, found: ASSUME at Line: 3, Column: 30")
```

In addition, in Clickhouse table constraint expressions aren't required to have top-level parenthesis. We should support these dialect attributes for Clickhouse.

Commencez par le chemin du parseur ClickHouseDialect, exercé par la commande `cargo run --features json_example --example cli assume_test.sql --clickhouse`. Utilisez l’exemple SQL de l’issue comme cas de parsing, et considérez le travail comme terminé lorsque les contraintes de table ASSUME sont correctement analysées avec et sans parenthèses de niveau supérieur.

Rédigé par le modèle d'indexation à partir du texte de l'issue.