[rust] Add a SnapshotId type alias

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss-rust/issues) and found nothing similar.

### Description

We have TableId, PartitionId, and BucketId aliases in lib.rs, but snapshot_id is still spelled i64 everywhere. Adding a matching SnapshotId alias would keep things consistent and make signatures easier to read.

A few hand-written sites to update: LakeSnapshot (metadata/table.rs), KvSnapshotLeaseForBucket (metadata/kv_snapshot_lease.rs), and the get_kv_snapshot_metadata / get_lake_snapshot request constructors.

### Willingness to contribute

- [ ] I'm willing to submit a PR!

Start in lib.rs to inspect the existing TableId, PartitionId, and BucketId aliases, then review SnapshotId uses in metadata/table.rs and metadata/kv_snapshot_lease.rs. Check the get_kv_snapshot_metadata and get_lake_snapshot request constructors; done means the named snapshot_id sites use a consistent SnapshotId alias.

Written by the indexing model from the issue text.