oraclewriter希望增加对mergeinto的支持

datax对mysql的insert into/replace into和insert into ... on duplicate key update都是支持的，但是对于oracle仅支持insert，项目中遇到主键冲突无法处理，希望增加对merge into的支持

No files, tests, or entry points are named. Start by locating the Oracle writer and its existing insert handling, then determine how merge-into support should fit the reported primary-key-conflict case. Done means Oracle writes handle the requested conflict behavior, with coverage for the relevant insert and conflict scenarios.

Written by the indexing model from the issue text.