ARROW-8177: [rust] Make schema_to_fb_offset public because it is very…

… useful!

I need this exact function in my application and it'd be a lot easier to just use it within Arrow instead of having to copy it out or recreate it!

Closes #6677 from maxburke/expose_schema_to_fb_offset and squashes the following commits:

52c1d85 <Max Burke> Make schema_to_fb_offset public because it is very useful!

Authored-by: Max Burke <max@urbanlogiq.com>
Signed-off-by: Paddy Horan <paddyhoran@hotmail.com>

rust/arrow/src/ipc/convert.rs

@@ -75,7 +75,7 @@ pub(crate) fn schema_to_fb(schema: &Schema) -> FlatBufferBuilder {
     fbb
 }
 
-pub(crate) fn schema_to_fb_offset<'a: 'b, 'b>(
+pub fn schema_to_fb_offset<'a: 'b, 'b>(
     mut fbb: &'a mut FlatBufferBuilder,
     schema: &Schema,
 ) -> WIPOffset<ipc::Schema<'b>> {