Add tuples_skipped to pg_stat_progress_copy

Hi,

132de9968840c introduced SAVE_ERROR_TO option to COPY and enabled to
skip malformed data, but there is no way to watch the number of skipped
rows during COPY.

Attached patch adds tuples_skipped to pg_stat_progress_copy, which
counts the number of skipped tuples because source data is malformed.
If SAVE_ERROR_TO is not specified, this column remains zero.

The advantage would be that users can quickly notice and stop COPYing
when there is a larger amount of skipped data than expected, for
example.

As described in commit log, it is expected to add more choices for
SAVE_ERROR_TO like 'log' and using such options may enable us to know
the number of skipped tuples during COPY, but exposed in
pg_stat_progress_copy would be easier to monitor.

What do you think?

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Regards,

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Atsushi Torikoshi
NTT DATA Group Corporation