Support table-level auto_flush_interval

### What type of enhancement is this? Configuration / User experience ### What does the enhancement do? Make `auto_flush_interval` configurable at the table (region) level, in addition to the existing global setting. Today `auto_flush_interval` is a global Mito engine config (`src/mito2/src/config.rs`, default 30min). It controls how long a region can stay un-flushed before the periodic flush job flushes it (see `flush_periodically` and `flush_regions_on_engine_full` in `src/mito2/src/worker/handle_flush.rs`, both read `self.config.auto_flush_interval`). A single global value doesn't fit every workload. In practice you often want a different flush cadence for a few specific tables while leaving the rest on the default: - Low-traffic tables that should flush sooner so data lands in object storage / becomes queryable with bounded latency. - High-traffic or throwaway tables where you want a longer interval to reduce small-file pressure and compaction load. Proposed behavior: - Add an `auto_flush_interval` table option (e.g. `WITH ('auto_flush_interval' = '5m')`), stored in the region options. - The periodic flush logic uses the per-region value when set, and falls back to the global engine config when unset. - Setting it via `ALTER TABLE` should take effect without restart, consistent with how other region options behave. This mirrors options like `ttl`, which already support both a global default and a per-table override. ### Implementation challenges - Add `auto_flush_interval: Option<Duration>` to `RegionOptions` (`src/mito2/src/region/options.rs`), with parsing/validation consistent with existing duration options. - In `handle_flush.rs`, resolve the effective interval per region: `region_options.auto_flush_interval.unwrap_or(self.config.auto_flush_interval)` instead of always using the global config. - Wire the option through SQL table options -> region create/alter requests so it can be set at `CREATE TABLE` and changed via `ALTER TABLE`. - Make sure the option is round-tripped in `SHOW CREATE TABLE` and covered by sqlness tests.