[Feature] Introduce prefix index for paimon

### Search before asking

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

### Motivation

Because the data of paimon is sorted, we can introduce a built-in prefix index to improve query performance, like max/min index

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

No files, tests, or entry points are named in the issue. Start by locating Paimon’s existing max/min index implementation and query path, then define what a built-in prefix index should cover and how its query-performance improvement will be verified.

Written by the indexing model from the issue text.