[Feature] Fluss client support to pin/unpin snapshot

### Search before asking

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

### Motivation

Currently, for primary key table, Fluss will read the kv snapshot. But it brings a problem that while reading the snapshot, we may expire some files in the snapshot which will cause snapshot reading fail.

Although user can set configuration `kv.snapshot.num-retained`(1 by default) to a larger value, it may not user friendly.

### Solution

Support fluss client to pin a snapshot so that we won't expire any files in this snapshot.

### Anything else?

_No response_

### Willingness to contribute

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

Start by locating the Fluss client APIs for reading primary-key table KV snapshots and the handling of kv.snapshot.num-retained. Trace how snapshot files are expired during reads, then determine the client-facing pin and unpin behavior and how completion should prevent expiration of the pinned snapshot.

Written by the indexing model from the issue text.