[Improvement] Support writing iceberg branchs

### What would you like to be improved?

Add related test cases in `SparkIcebergIT` and update doc.

### How should we improve?

referred to (https://iceberg.apache.org/docs/latest/spark-writes/#writing-to-branches)

Start by reading the SparkIcebergIT integration test and the linked Apache Iceberg documentation on writing to branches. Add the related test cases there and update the project documentation as requested; done means branch writes are covered and the relevant documentation reflects the supported behavior.

Written by the indexing model from the issue text.