Support Clickhouse ASSUME table constraints (and remove required parenthetical)

[pelovett]

Hello, currently Clickhouse's ASSUME table constraints generate parse errors:

$ cat assume_test.sql
CREATE TABLE my_table (
    `date` Date,
    CONSTRAINT my_constraint ASSUME (date > '2025-01-01')
)
ENGINE = MergeTree
;

---

$ 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.