PG_ENSURE_ERROR_CLEANUP and nested blocks

Hi,

If you use PG_ENSURE_ERROR_CLEANUP with nested blocks, the compiler warns about
shadow locals. There is no harm since each PG_TRY and PG_CATCH uses its own
context stack. The current code doesn't have an example of such usage but
pglogical [1] does. I don't know if other extensions have the same issue too.

The PG_TRY and PG_CATCH already accept an optional parameter (commit
112f0225dbfe) to avoid this exact issue. Since PG_ENSURE_ERROR_CLEANUP /
PG_END_ENSURE_ERROR_CLEANUP already use PG_TRY / PG_CATCH, add an optional
parameter too. (I opted to a variadic argument argument -- mimicking how PG_TRY
already handles its optional suffix parameter -- because it avoids changing existing callers).

[1] https://github.com/2ndQuadrant/pglogical/issues/521#issuecomment-4925354899

--
Euler Taveira
EDB https://www.enterprisedb.com/