[Feature] Expose Flink Source in FlinkSourceBuilder API

### Search before asking

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

### Motivation

As a user of the `FlinkSourceBuilder` I'd like to be able to get a `org.apache.flink.api.connector.source.Source` instance returned instead of only having access to the `DataStream` via `build()` and `buildForRow()`.

In my particular case I want to bootstrap my pipeline with historical data from a bounded source using Flink's `HybridSource`, and have the streaming data from a Paimon table. In order to do so, I need to provide the `HybridSource` with a `Source` instance which currently isn't possible using the `FlinkSourceBuilder`.

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Locate the FlinkSourceBuilder API and inspect build() and buildForRow(), then trace how they create the DataStream and underlying source. Expose a Source instance that can be supplied to Flink's HybridSource while preserving the existing builder behavior; verify the resulting API with the project's relevant tests.

Written by the indexing model from the issue text.