Add wait events for COPY file/program operations

[NikolayS]

Summary

COPY operations previously showed wait_event = NULL when blocking on file/program I/O, making it impossible to diagnose performance issues during data loading, ETL operations, or pg_dump/pg_restore.

This patch adds two new IO wait events:

• COPY_DATA_READ: When COPY FROM blocks reading from file or program
• COPY_DATA_WRITE: When COPY TO blocks writing to file or program

Coverage

COPY Variant	Wait Event
COPY FROM file	COPY_DATA_READ
COPY TO file	COPY_DATA_WRITE
COPY FROM PROGRAM	COPY_DATA_READ
COPY TO PROGRAM	COPY_DATA_WRITE
COPY FROM/TO STDIN/STDOUT	CLIENT_READ/WRITE (existing)

Note: STDIN/STDOUT already have coverage via Client/ClientRead and Client/ClientWrite events at the protocol layer.

Use Cases

• Diagnose slow pg_dump/pg_restore operations
• Identify storage I/O bottlenecks during bulk loads
• Analyze pipe buffer congestion in ETL pipelines
• ASH (Active Session History) analysis of COPY operations

Changes

• src/backend/commands/copyfromparse.c: Wrap fread() with wait events
• src/backend/commands/copyto.c: Wrap fwrite() with wait events
• src/backend/utils/activity/wait_event_names.txt: Add event definitions