[Feature] It may be a memory bottleneck when read all manifests.

### Search before asking

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

### Motivation

Snapshot.dataManifests() will read all the ManifestFileMeta in memory . If there are too many files in a table ,it may cause oom. And when ManifestFileMeta.merge() to merge the manifests , the merge map is also a memory container , which will case the same problem.
I think we can use a spillable container to save manifests .

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start by tracing Snapshot.dataManifests() and ManifestFileMeta.merge(), the entry points named in the issue, to see where all manifest metadata and the merge map are held in memory. Done should address the reported out-of-memory risk for tables with many files; the issue names no specific files or tests.

Written by the indexing model from the issue text.