[server] Add authorization to Snapshot Read Operations (getLatestKvSnapshots, getKvSnapshotMetadata, getLatestLakeSnapshot)

### Search before asking

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

### Motivation

**Part of umbrella issue #2007 - Phase 1: External RPCs**

Add authorization checks for snapshot read operations:
- `getLatestKvSnapshots` - TABLE/DESCRIBE
- `getKvSnapshotMetadata` - TABLE/DESCRIBE
- `getLatestLakeSnapshot` - TABLE/DESCRIBE

These are client-facing snapshot metadata operations that need authorization to
prevent unauthorized access to snapshot information.

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

Locate the server entry points for getLatestKvSnapshots, getKvSnapshotMetadata, and getLatestLakeSnapshot, then read the authorization handling for related external RPCs. Add TABLE/DESCRIBE checks to these three snapshot read operations and verify that authorized and unauthorized requests behave as intended.

Written by the indexing model from the issue text.