Add new COPY option LOG_VERBOSITY.

This commit adds a new COPY option LOG_VERBOSITY, which controls the
amount of messages emitted during processing. Valid values are
'default' and 'verbose'.

This is currently used in COPY FROM when ON_ERROR option is set to
ignore. If 'verbose' is specified, a NOTICE message is emitted for
each discarded row, providing additional information such as line
number, column name, and the malformed value. This helps users to
identify problematic rows that failed to load.

Author: Bharath Rupireddy
Reviewed-by: Michael Paquier, Atsushi Torikoshi, Masahiko Sawada
Discussion: https://www.postgresql.org/message-id/CALj2ACUk700cYhx1ATRQyRw-fBM%2BaRo6auRAitKGff7XNmYfqQ%40mail.gmail.com

src/backend/parser/gram.y

@@ -3530,6 +3530,7 @@ copy_generic_opt_arg:
 			opt_boolean_or_string			{ $$ = (Node *) makeString($1); }
 			| NumericOnly					{ $$ = (Node *) $1; }
 			| '*'							{ $$ = (Node *) makeNode(A_Star); }
+			| DEFAULT                       { $$ = (Node *) makeString("default"); }
 			| '(' copy_generic_opt_arg_list ')'		{ $$ = (Node *) $2; }
 			| /* EMPTY */					{ $$ = NULL; }
 		;