Rework some code handling pg_subscription data in psql and pg_dump

Rework some code handling pg_subscription data in psql and pg_dump

This commit fixes some inconsistencies found in the frontend code when
dealing with subscription catalog data.

The following changes are done:
- pg_subscription.h gains a EXPOSE_TO_CLIENT_CODE, so as more content
defined in pg_subscription.h becomes available in pg_subscription_d.h
for the frontend.
- In psql's describe.c, substream can be switched to use CppAsString2()
with its three LOGICALREP_STREAM_* values, with pg_subscription_d.h
included.
- pg_dump.c included pg_subscription.h, which is a header that should
only be used in the backend code.  The code is updated to use
pg_subscription_d.h instead.
- pg_dump stored all the data from pg_subscription in SubscriptionInfo
with only strings, and a good chunk of them are boolean and char values.
Using strings is not necessary, complicates the code (see for example
two_phase_disabled[] removed here), and is inconsistent with the way
other catalogs' data is handled.  The fields of SubscriptionInfo are
reordered to match with the order in its catalog, while on it.

Reviewed-by: Hayato Kuroda
Discussion: https://postgr.es/m/Z0lB2kp0ksHgmVuk@paquier.xyz