Moving Java external transform from core-construction-java to sdks-java-core #19921
Description
Creating a separate issue from BEAM-6747.
Related discusstion: https://lists.apache.org/list.html?dev@beam.apache.org:lte=111M:How%20to%20expose%2Fuse%20the%20External%20transform%20on%20Java%20SDK
Imported from Jira BEAM-8546. Original Jira may contain additional context.
Reported by: heejong.