Adding a '--clean-publisher-objects' option to 'pg_createsubscriber' utility.

Hi all,

I propose adding the --clean-publisher-objects option to the
pg_createsubscriber utility. As discussed in [1], this feature ensures
a clean and streamlined setup of logical replication by removing stale
or unnecessary publications from the subscriber node. These
publications, replicated during streaming replication, become
redundant after converting to logical replication and serve no further
purpose. This patch introduces the drop_all_publications() function,
which efficiently fetches and drops all publications on the subscriber
node within a single transaction. Since this cleanup is not required
when upgrading streaming replication clusters, as mentioned in [2],
this feature is supported only when the --clean-publisher-objects
option is specified, allowing users to choose accordingly.
Additionally, other related objects, such as subscriptions and
replication slots, may also require cleanup. I plan to analyze this
further and include them in subsequent patches.
The attached patch includes the necessary changes for this feature.

[1] - https://www.postgresql.org/message-id/CAExHW5t4ew7ZrgcDdTv7YmuG7LVQT1ZaEny_EvtngHtEBNyjcQ%40mail.gmail.com
[2] - https://amitkapila16.blogspot.com/2024/09/online-upgrading-logical-and-physical.html

Thanks and regards,
Shubham Khanna.