[lake/lance] Support Union Read query against Lance table

### Search before asking

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

### Motivation

Batch / Union Read query against Lance table is [currently unimplemented](https://github.com/apache/fluss/blob/575db63687793be0086c63f6793bce683e1281f0/fluss-lake/fluss-lake-lance/src/main/java/org/apache/fluss/lake/lance/LanceLakeStorage.java#L49)

```
public class LanceLakeStorage implements LakeStorage {
...
@Override
public LakeSource createLakeSource(TablePath tablePath) {
throw new UnsupportedOperationException("Not implemented");
}
}
```

Implementing LakeSource here will allow batch / union read from Flink SQL.

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

Start in fluss-lake/fluss-lake-lance/src/main/java/org/apache/fluss/lake/lance/LanceLakeStorage.java at createLakeSource, then inspect the LakeSource interface and related lake storage implementations. Determine the requirements for batch and union reads from Flink SQL, and consider the work complete when LanceLakeStorage provides a working LakeSource for those queries.

Written by the indexing model from the issue text.