Merge pull request #22755 from zhangsw/feature-rgw-appendobj

rgw: Add append object api.

diff --cc src/rgw/rgw_common.cc
Simple merge

diff --cc src/rgw/rgw_common.h
Simple merge

diff --cc src/rgw/rgw_op.cc
Simple merge

diff --cc src/rgw/rgw_rados.cc
Simple merge