[SPARK-51763] Support struct type in ArrowReader

### What changes were proposed in this pull request?

This PR aims to support `struct` type in `ArrowReader`.

### Why are the changes needed?

`ArrowReader` depends on `ArrowReaderHelper` which depends on `ProtoUtil.fromProto` function. This PR extends `fromProto` to handle `struct` type in order to avoid unknown type error in `ArrowReader` . The following is the result of `Scala` client.

```
$ bin/spark-connect-shell --remote sc://localhost:15002
...
Welcome to
      ____              __
     / __/__  ___ _____/ /__
    _\ \/ _ \/ _ `/ __/  '_/
   /___/ .__/\_,_/_/ /_/\_\   version 4.0.0-preview2
      /_/

Type in expressions to have them evaluated.
Spark session available as 'spark'.

scala> sql("SELECT struct(1), struct(2, struct(3))").show()
+---------+--------------------+
|struct(1)|struct(2, struct(3))|
+---------+--------------------+
|      {1}|            {2, {3}}|
+---------+--------------------+
```

### Does this PR introduce _any_ user-facing change?

No. There is no behavior change.

### How was this patch tested?

Pass the CIs with the newly added test cases.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #50 from dongjoon-hyun/SPARK-51763.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

Sources/SparkConnect/ProtoUtil.swift

@@ -64,6 +64,8 @@ func fromProto(  // swiftlint:disable:this cyclomatic_complexity
       let arrowUnit: ArrowTime64Unit = timeType.unit == .microsecond ? .microseconds : .nanoseconds
       arrowType = ArrowTypeTime64(arrowUnit)
     }
+  case .struct_:
+    arrowType = ArrowType(ArrowType.ArrowStruct)
   default:
     arrowType = ArrowType(ArrowType.ArrowUnknown)
   }

Tests/SparkConnectTests/Resources/queries/struct.sql

@@ -0,0 +1 @@
+SELECT struct(1), struct(2, struct(3))

Tests/SparkConnectTests/Resources/queries/struct.sql.json

@@ -0,0 +1 @@
+[["{1}","{2,{3}}"]]