postgres_fdw: Extend postgres_fdw_get_connections to return remote backend PID.

postgres_fdw: Extend postgres_fdw_get_connections to return remote backend PID.

This commit adds a new "remote_backend_pid" output column to
the postgres_fdw_get_connections function. It returns the process ID of
the remote backend, on the foreign server, handling the connection.

This enhancement is useful for troubleshooting, monitoring, and reporting.
For example, if a connection is unexpectedly closed by the foreign server,
the remote backend's PID can help diagnose the cause.

No extension version bump is needed, as commit c297a47c5f already
handled it for v18~.

Author: Sagar Dilip Shedge <sagar.shedge92@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAPhYifF25q5xUQWXETfKwhc0YVa_6+tfG9Kw4bCvCjpCWxYs2A@mail.gmail.com