[PATCH v1] Add pg_stat_progress overview view

Hi,

finding active progress reports currently requires querying each
pg_stat_progress_* view separately.

This patch adds pg_stat_progress with three columns: pid, command, and
progress_view, pointing to the detailed view for each operation. It
reuses the existing infrastructure without backend changes. IMHO,
these are the minimal common fields. I also considered datid and
datname, but some tracked operations, such as base backups, cover the
whole cluster.

Documentation and tests are included. Build, regression, isolation,
and new TAP tests passed locally.

I'd appreciate feedback on the usefulness and proposed columns. My
longer-term aim is to make cluster operations easier to discover and
monitor, and to extend progress reporting to more commands.

Regards,
Josef