[Improvement] Introduce storage manager selection strategy for MultipleStorageManager

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

### Search before asking

- [X] I have searched in the [issues](https://github.com/apache/incubator-uniffle/issues?q=is%3Aissue) and found no similar issues.

### What would you like to be improved?

In current codebase, it has introduced the storageManager fallback strategy. But I think it's not general.

For example: I want to use the coldStorageManager directly for some specific apps, which are low priority or own a huge partition.

So I think the selection strategy will solve above problems, and at the same time, it also could cover the fallback mechanism.

### How should we improve?

_No response_

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

Start by reading MultipleStorageManager and the existing storage-manager fallback strategy. Trace how app identity and partition size can guide selection, then define the desired behavior for cold storage and fallback cases. Done means the strategy supports targeted app or large-partition selection while preserving fallback behavior.

Written by the indexing model from the issue text.