Add log_verbosity = 'silent' support to COPY command.

Add log_verbosity = 'silent' support to COPY command.

Previously, when the on_error option was set to ignore, the COPY command
would always log NOTICE messages for input rows discarded due to
data type incompatibility. Users had no way to suppress these messages.

This commit introduces a new log_verbosity setting, 'silent',
which prevents the COPY command from emitting NOTICE messages
when on_error = 'ignore' is used, even if rows are discarded.
This feature is particularly useful when processing malformed files
frequently, where a flood of NOTICE messages can be undesirable.

For example, when frequently loading malformed files via the COPY command
or querying foreign tables using file_fdw (with an upcoming patch to
add on_error support for file_fdw), users may prefer to suppress
these messages to reduce log noise and improve clarity.

Author: Atsushi Torikoshi
Reviewed-by: Masahiko Sawada, Fujii Masao
Discussion: https://postgr.es/m/ab59dad10490ea3734cf022b16c24cfd@oss.nttdata.com