BatchElements transform for Java SDK (#38369)

* skeleton

* calculate batch size

* global window batching

* window awae batching

* unit tests

* java api docs

* gemini comments

* checkstyle

* fix spotbug

* update doc and changes.md

* checkstyle

* whitespace

---------

Co-authored-by: Ganeshsivakumar <ganeshsivakumar85@gmail.com>

Author: ganesh-skumar

CHANGES.md

@@ -77,6 +77,7 @@
   compatible. Both coders can decode encoded bytes from the other coder
   ([#38139](https://github.com/apache/beam/issues/38139)).
 * (Python) Added type alias for with_exception_handling to be used for typehints. ([#38173](https://github.com/apache/beam/issues/38173)).
+* (Java) BatchElements transform for Java SDK ([#38369](https://github.com/apache/beam/issues/38369))
 * Added plugin mechanism to support different Lineage implementations (Java) ([#36790](https://github.com/apache/beam/issues/36790)).
 * (Python) Supported Python user type in Beam SQL. For example, SQL statements like `SELECT some_field from PCOLLECTION` can now operate a PCollection of Beam Row containing pickable Python user type ([#20738](https://github.com/apache/beam/issues/20738)).
 * (Python) Introduced `beam.coders.registry.register_row` as preferred API to register a named tuple or dataclass with a Beam Row. At pipelne runtime, the original type associated with the registered row are preserved across the serialization boundary ([#38108](https://github.com/apache/beam/issues/38108)).
@@ -2435,4 +2436,4 @@ Schema Options, it will be removed in version `2.23.0`. ([BEAM-9704](https://iss
 
 ## Highlights
 
-- For versions 2.19.0 and older release notes are available on [Apache Beam Blog](https://beam.apache.org/blog/).
+- For versions 2.19.0 and older release notes are available on [Apache Beam Blog](https://beam.apache.org/blog/).