[rust] Add a SnapshotId type alias

[fresh-borzoni]

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• I searched in the 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!