Support ALTER SUBSCRIPTION ... ADD/DROP PUBLICATION ... syntax

Hi, hackers

When I read the discussion in [1], I found that update subscription's publications
is complicated.

For example, I have 5 publications in subscription.

    CREATE SUBSCRIPTION mysub1 CONNECTION 'host=localhost port=5432 dbname=postgres'
    PUBLICATION mypub1, mypub2, mypub3, mypub4, mypub5;

If I want to drop "mypub4", we should use the following command:

    ALTER SUBSCRIPTION mysub1 SET PUBLICATION mypub1, mypub2, mypub3, mypub5;

Also, if I want to add "mypub7" and "mypub8", it will use:

    ALTER SUBSCRIPTION mysub1 SET PUBLICATION mypub1, mypub2, mypub3, mypub5, mypub7, mypub8;

Attached implement ALTER SUBSCRIPTION ... ADD/DROP PUBLICATION ... syntax, for the above
two cases, we can use the following:

    ALTER SUBSCRIPTION mysub1 DROP PUBLICATION mypub4;

    ALTER SUBSCRIPTION mysub1 DROP PUBLICATION mypub7, mypub8;

I think it's more convenient. Any thoughts?

[1] -
https://www.postgresql.org/message-id/MEYP282MB16690CD5EC5319FC35B2F78AB6BD0%40MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM

-- 
Regrads,
Japin Li.
ChengDu WenWu Information Technology Co.,Ltd.