[spark] xgboost.spark supports saving model that is very large

Currently, ` xgboost.spark ` saves model by:

```
booster = xgb_model.get_booster().save_raw("json").decode("utf-8")
_get_spark_session().sparkContext.parallelize([booster], 1).saveAsTextFile(
model_save_path
)
```

when `booster` is a string larger than 2GB, spark will raise error like:
```
java.lang.OutOfMemoryError: Requested array size exceeds VM limit
at java.lang.StringCoding.encode(StringCoding.java:350)
at java.lang.String.getBytes(String.java:941)
at org.apache.spark.unsafe.types.UTF8String.fromString(UTF8String.java:163)
at org.apache.spark.sql.execution.python.EvaluatePython$$anonfun$$nestedInanonfun$makeFromJava$11$1.applyOrElse(EvaluatePython.scala:149)
at org.apache.spark.sql.execution.python.EvaluatePython$.nullSafeConvert(EvaluatePython.scala:213)
at org.apache.spark.sql.execution.python.EvaluatePython$.$anonfun$makeFromJava$11(EvaluatePython.scala:148)
at org.apache.spark.sql.execution.python.EvaluatePython$$$Lambda$8923/940497638.apply(Unknown Source)
at org.apache.spark.sql.execution.python.EvaluatePython$$anonfun$$nestedInanonfun$makeFromJava$16$1.applyOrElse(EvaluatePython.scala:195)
at org.apache.spark.sql.execution.python.EvaluatePython$.nullSafeConvert(EvaluatePython.scala:213)
at org.apache.spark.sql.execution.python.EvaluatePython$.$anonfun$makeFromJava$16(EvaluatePython.scala:182)
at org.apache.spark.sql.execution.python.EvaluatePython$$$Lambda$8925/1092357589.apply(Unknown Source)
at org.apache.spark.sql.SparkSession.$anonfun$applySchemaToPythonRDD$2(SparkSession.scala:960)
at org.apache.spark.sql.SparkSession$$Lambda$8929/213457688.apply(Unknown Source)
```

We need to consider split the model string and save it.

Start from the xgboost.spark save path shown in the issue, where save_raw("json") is parallelized and written with saveAsTextFile. Reproduce or trace the >2GB failure, then determine a storage approach that avoids a single oversized string and verify large-model save and load behavior.

Written by the indexing model from the issue text.