add column (query_start timestamptz) to progress report views

hi.

for all the progress report views[1],
we can add a timestamptz column, maybe named as query_start.

People generally want to track the progress of operations and the time spent on
them. While pg_stat_activity already provides column query_start, including
query_start in pg_stat_get_progress_info would eliminate the need
for JOIN with pg_stat_activity in some cases.
Overall it will make the progress report monitoring more convenient, IMHO.

as of implementation: we can occupy pg_stat_get_progress_info last parameter
param20 for column query_start.

what do you think?

[1]: https://www.postgresql.org/docs/current/progress-reporting.html#COPY-PROGRESS-REPORTING
[2] https://www.postgresql.org/docs/current/monitoring-stats.html#MONITORING-PG-STAT-ACTIVITY-VIEW