ARROW-8380 Export StringDictionaryBuilder from arrow::array crate

Closes #6881 from jhorstmann/ARROW-8380-export-string-dictionary-builder

Authored-by: Jörn Horstmann <joern.horstmann@signavio.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>

rust/arrow/src/array/mod.rs

@@ -190,6 +190,7 @@ pub use self::builder::ListBuilder;
 pub use self::builder::PrimitiveBuilder;
 pub use self::builder::PrimitiveDictionaryBuilder;
 pub use self::builder::StringBuilder;
+pub use self::builder::StringDictionaryBuilder;
 pub use self::builder::StructBuilder;
 
 pub type BooleanBuilder = PrimitiveBuilder<BooleanType>;