libpqsrv_cancel() currently always reports the generic PG_WAIT_CLIENT wait event while waiting for a cancel request to be acknowledged by the remote server, no matter which caller invoked it. This makes it hard to tell from pg_stat_activity whether a backend is stuck waiting on a cancel ack versus any other generic client wait.
Attached patch adds a wait_event_info parameter to libpqsrv_cancel(), so each caller can report a wait event specific to its own operation, following the same pattern already used for connection and query-result waits (e.g. PostgresFdwConnect, PostgresFdwGetResult). postgres_fdw now reports PostgresFdwCancel, and dblink now reports DblinkCancel, each allocated lazily via WaitEventExtensionNew() like their sibling wait events. Documentation for both new wait events is included.
The new wait event is documented in postgres-fdw.sgml.
