## What changes were proposed in this pull request?
This PR proposes two things.
1.. Deduplicates the type checking logic. While I am here, I checked each type. Currently, binary type, float type, nested struct type and array type are not supported.
**For map and nested struct types:**
it's expected to be unsupported per Spark's arrow optimization.
```
Exception in thread "serve-Arrow" java.lang.UnsupportedOperationException: Unsupported data type: map<string,double>
...
```
```
Exception in thread "serve-Arrow" java.lang.UnsupportedOperationException: Unsupported data type: struct<type:tinyint,size:int,indices:array<int>,values:array<double>>
...
```
Please track the trace below to double check.
```
at org.apache.spark.sql.execution.arrow.ArrowUtils$.toArrowType(ArrowUtils.scala:56)
at org.apache.spark.sql.execution.arrow.ArrowUtils$.toArrowField(ArrowUtils.scala:92)
at org.apache.spark.sql.execution.arrow.ArrowUtils$.$anonfun$toArrowSchema$1(ArrowUtils.scala:116)
at scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:237)
at scala.collection.Iterator.foreach(Iterator.scala:941)
at scala.collection.Iterator.foreach$(Iterator.scala:941)
at scala.collection.AbstractIterator.foreach(Iterator.scala:1429)
at scala.collection.IterableLike.foreach(IterableLike.scala:74)
at scala.collection.IterableLike.foreach$(IterableLike.scala:73)
at org.apache.spark.sql.types.StructType.foreach(StructType.scala:99)
at scala.collection.TraversableLike.map(TraversableLike.scala:237)
at scala.collection.TraversableLike.map$(TraversableLike.scala:230)
at org.apache.spark.sql.types.StructType.map(StructType.scala:99)
at org.apache.spark.sql.execution.arrow.ArrowUtils$.toArrowSchema(ArrowUtils.scala:115)
at org.apache.spark.sql.execution.arrow.ArrowBatchStreamWriter.<init>(ArrowConverters.scala:50)
at org.apache.spark.sql.Dataset.$anonfun$collectAsArrowToR$2(Dataset.scala:3215)
at org.apache.spark.sql.Dataset.$anonfun$collectAsArrowToR$2$adapted(Dataset.scala:3212)
```
**For float and binary types:**
They cause corrupt values in some cases. It needs to be investigated separately.
**For array type:**
```
Error in Table__to_dataframe(x, use_threads = use_threads) :
cannot handle Array of type list
```
Seems to be Arrow's R library limitation. It needs to be investigated separately as well.
2.. While I am touching the type specification codes across Arrow optimization, I move the Arrow optimization related tests into a separate filed called `test_arrow.R`.
## How was this patch tested?
Tests were added and also manually tested.
Closes#23969 from HyukjinKwon/SPARK-26920.
Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>
93ea353cae