Parquet: Make MetadataLoader public (#5137)

* Make  public

* revert change in MetadataFetchFn

Author: AdamGS

parquet/src/arrow/async_reader/metadata.rs

@@ -28,7 +28,7 @@ use std::future::Future;
 use std::ops::Range;
 
 /// A data source that can be used with [`MetadataLoader`] to load [`ParquetMetaData`]
-pub(crate) trait MetadataFetch {
+pub trait MetadataFetch {
     fn fetch(&mut self, range: Range<usize>) -> BoxFuture<'_, Result<Bytes>>;
 }
 
@@ -39,9 +39,7 @@ impl<'a, T: AsyncFileReader> MetadataFetch for &'a mut T {
 }
 
 /// An asynchronous interface to load [`ParquetMetaData`] from an async source
-///
-/// Crate-private until stabilised
-pub(crate) struct MetadataLoader<F> {
+pub struct MetadataLoader<F> {
     /// Function that fetches byte ranges asynchronously
     fetch: F,
     /// The in-progress metadata