pgsql: CREATE SUBSCRIPTION: do not construct conninfo unnecessarily.

CREATE SUBSCRIPTION: do not construct conninfo unnecessarily.

Still check that the creating user has USAGE privileges on the server,
and that the FDW supports subscription connections.

Addresses finding 1 in the report from the linked discussion.

Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Discussion: https://postgr.es/m/20260710195902.4f.noahmisch%40microsoft.com
Discussion: https://postgr.es/m/e103ae8daf74485e0c0ebde297fae735d38f54d1.camel@j-davis.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/254beb174e61e339c43e1096053b0a0a2e9cf856

Modified Files
--------------
src/backend/commands/subscriptioncmds.c | 37 ++++++++++++++++++++++--------
src/backend/foreign/foreign.c | 4 ++--
src/test/regress/expected/subscription.out | 4 ++--
3 files changed, 31 insertions(+), 14 deletions(-)