background

Task

• add a system variable in tidb and txn_source in kvcontext variable: add txn_source into kv.context #39159
• add a new field txn_source in kvprotobuf add txn_source in kv context kvproto#1010
• a new field txn_source in write_cf FD: Bi-Directional Replication for ticdc tikv/tikv#13779