Add strict_multi_assignment and too_many_rows plpgsql checks

Add strict_multi_assignment and too_many_rows plpgsql checks

Until now shadowed_variables was the only plpgsql check supported by
plpgsql.extra_warnings and plpgsql.extra_errors.  This patch introduces
two new checks - strict_multi_assignment and too_many_rows.  Unlike
shadowed_variables, these new checks are enforced at run-time.

strict_multi_assignment checks that commands allowing multi-assignment
(for example SELECT INTO) have the same number of sources and targets.
too_many_rows checks that queries with an INTO clause return one row
exactly.

These checks are aimed at cases that are technically valid and allowed,
but are often a sign of a bug.  Therefore those checks are expected to
be enabled primarily in development and testing environments.

Author: Pavel Stehule
Reviewed-by: Stephen Frost, Tomas Vondra
Discussion: https://www.postgresql.org/message-id/flat/CAFj8pRA2kKRDKpUNwLY0GeG1OqOp+tLS2yQA1V41gzuSz-hCng@mail.gmail.com