Add IO wait events for COPY file/program operations

Add two new IO wait events:
- COPY_DATA_READ: COPY FROM blocking on file or program read
- COPY_DATA_WRITE: COPY TO blocking on file or program write

This enables diagnosing:
- Storage I/O bottlenecks during bulk loads (COPY FROM '/path/to/file')
- Slow exports to files (COPY TO '/path/to/file')
- Pipe buffer congestion in ETL pipelines (COPY FROM/TO PROGRAM)

COPY FROM/TO STDIN/STDOUT already have coverage via Client/ClientRead
and Client/ClientWrite at the protocol layer.

These events are distinct from the existing COPY_FILE_READ/WRITE events,
which instrument file-to-file copy operations in basebackup code.

Author: NikolayS

src/backend/commands/copyfromparse.c

@@ -249,7 +249,9 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 	switch (cstate->copy_src)
 	{
 		case COPY_FILE:
+			pgstat_report_wait_start(WAIT_EVENT_COPY_DATA_READ);
 			bytesread = fread(databuf, 1, maxread, cstate->copy_file);
+			pgstat_report_wait_end();
 			if (ferror(cstate->copy_file))
 				ereport(ERROR,
 						(errcode_for_file_access(),

src/backend/commands/copyto.c

@@ -454,6 +454,7 @@ CopySendEndOfRow(CopyToState cstate)
 	switch (cstate->copy_dest)
 	{
 		case COPY_FILE:
+			pgstat_report_wait_start(WAIT_EVENT_COPY_DATA_WRITE);
 			if (fwrite(fe_msgbuf->data, fe_msgbuf->len, 1,
 					   cstate->copy_file) != 1 ||
 				ferror(cstate->copy_file))
@@ -486,6 +487,7 @@ CopySendEndOfRow(CopyToState cstate)
 							(errcode_for_file_access(),
 							 errmsg("could not write to COPY file: %m")));
 			}
+			pgstat_report_wait_end();
 			break;
 		case COPY_FRONTEND:
 			/* Dump the accumulated row as one CopyData message */

src/backend/utils/activity/wait_event_names.txt

@@ -210,6 +210,8 @@ CONTROL_FILE_SYNC	"Waiting for the <filename>pg_control</filename> file to reach
 CONTROL_FILE_SYNC_UPDATE	"Waiting for an update to the <filename>pg_control</filename> file to reach durable storage."
 CONTROL_FILE_WRITE	"Waiting for a write to the <filename>pg_control</filename> file."
 CONTROL_FILE_WRITE_UPDATE	"Waiting for a write to update the <filename>pg_control</filename> file."
+COPY_DATA_READ	"Waiting for a read from a file or program during COPY FROM."
+COPY_DATA_WRITE	"Waiting for a write to a file or program during COPY TO."
 COPY_FILE_COPY	"Waiting for a file copy operation."
 COPY_FILE_READ	"Waiting for a read during a file copy operation."
 COPY_FILE_WRITE	"Waiting for a write during a file copy operation."