[Feature] snapshot should be interface.

### Search before asking

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

### Motivation

`org.apache.paimon.Snapshot` was used by anywhere but it's class but not interface. It's function like `public List dataManifests(ManifestList manifestList)` is not interface eg. `ManifestFileMeta` should be interface eg.

We are writing a version of paimon that not based file, we have to change this classes for api compatible.

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Locate org.apache.paimon.Snapshot and ManifestFileMeta, then inspect their usages and existing public API constraints. The work is done when the relevant classes expose interface-based APIs that support a non-file-based Paimon implementation while remaining API-compatible.

Written by the indexing model from the issue text.