feat(reader): Make ArrowReaderBuilder::new public (apache#1748)

## Which issue does this PR close?


- Part of apache#1749.

## What changes are included in this PR?

- Change `ArrowReaderBuilder::new` to be `pub` instead of `pub(crate)`.

## Are these changes tested?

- No new tests for this. Currently being used in DataFusion Comet:
apache/datafusion-comet#2528

Author: mbutrovich

crates/iceberg/src/arrow/reader.rs

@@ -68,7 +68,7 @@ pub struct ArrowReaderBuilder {
 
 impl ArrowReaderBuilder {
     /// Create a new ArrowReaderBuilder
-    pub(crate) fn new(file_io: FileIO) -> Self {
+    pub fn new(file_io: FileIO) -> Self {
         let num_cpus = available_parallelism().get();
 
         ArrowReaderBuilder {