Rework subscription-related code for psql and pg_dump

Hi all,

While looking at some of the code related to subscriptions in psql,
coming down to make LOGICALREP_STREAM_* available for client code,
I've also noticed quite a few inconsistencies and mistakes with how
pg_subscription is handled in pg_dump, that have accumulated over the
years as of what looks like set of copy-pastos for most of them:
- pg_dump.c includes pg_subscription.h and not pg_subscription_d.h.
This is a mistake as the former should only be included in the backend
code.
- SubscriptionInfo has accumulated dust over the years, using
declarations types that don't map with its catalog. To keep it short
here, all the fields use (char *) leading to more DIY logic in the
code (see two_phase_disabled[]), while most of the fields are booleans
or char values. Switching to char values allows direct comparisons
with the contents of pg_subscription_d.h, leading to more consistent
code.
- Inconsistent position of fields between SubscriptionInfo and the
catalog pg_subscription.

EXPOSE_TO_CLIENT_CODE has been added to pg_subscription.h so as values
for substream, twophasestate and origin can be used directly in psql
and pg_dump, switching these to use pg_subscription_d.h as this is
client code.

While all that addressed, I am finishing with the patch attached.

Thoughts or comments?
--
Michael