Fix issue with EVENT TRIGGERS and ALTER PUBLICATION

Fix issue with EVENT TRIGGERS and ALTER PUBLICATION

When processing the "publish" options of an ALTER PUBLICATION command,
we call SplitIdentifierString() to split the options into a List of
strings.  Since SplitIdentifierString() modifies the delimiter
character and puts NULs in their place, this would overwrite the memory
of the AlterPublicationStmt.  Later in AlterPublicationOptions(), the
modified AlterPublicationStmt is copied for event triggers, which would
result in the event trigger only seeing the first "publish" option
rather than all options that were specified in the command.

To fix this, make a copy of the string before passing to
SplitIdentifierString().

Here we also adjust a similar case in the pgoutput plugin.  There's no
known issues caused by SplitIdentifierString() here, so this is being
done out of paranoia.

Thanks to Henson Choi for putting together an example case showing the
ALTER PUBLICATION issue.

Author: sunil s <sunilfeb26@gmail.com>
Reviewed-by: Henson Choi <assam258@gmail.com>
Reviewed-by: zengman <zengman@halodbtech.com>
Backpatch-through: 14