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.