[spark] Umbrella issue for scan optimization

### Search before asking

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

### Motivation

This is is an umbrella issue to track the basic scan optimization for spark connector.

- [x] #3117
- [ ] pk table predicate pushdown
- [x] #3239
- [x] #3241
- [ ] column pruning
- [x] #3215
- [ ] batch read instead of row read

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

Start by reading the linked issues #3117, #3239, #3241, and #3215 to understand the Spark connector scan optimization work already tracked. The remaining scope is primary-key table predicate pushdown, column pruning, and batch reads instead of row reads; done means the applicable unchecked items are completed and verified.

Written by the indexing model from the issue text.