[Feature] Introduce a bulk insert mode for dynamic bucket

### Search before asking

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

### Motivation

Remove bucket assigner node.

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

The issue names no files, tests, or entry points, and provides no implementation details beyond removing the bucket assigner node for dynamic buckets. Start by locating the dynamic-bucket and bulk-insert paths in the repository; done should mean the proposed mode works without that node and has coverage for the affected behavior.

Written by the indexing model from the issue text.