[Feature] Support fixed-partition sink for AppendOnly table

### Search before asking

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

### Motivation

When we use paimon as a message queue, records do not need to be partitioned by bucketKey to ensure the order in some scenarios. So we can support fixed-partition like kafka sink.

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start by locating the existing AppendOnly table sink and its bucketKey-based partitioning, then compare that flow with the fixed-partition behavior requested for a Kafka-like message queue. The issue provides no files or tests; done requires an agreed design and coverage showing records can use fixed partitions without relying on bucketKey ordering.

Written by the indexing model from the issue text.