pgsql: pg_dump: Fix dumping of security labels on subscriptions and eve

pg_dump: Fix dumping of security labels on subscriptions and event triggers.

Previously, pg_dump incorrectly queried pg_seclabel to retrieve security labels
for subscriptions, which are stored in pg_shseclabel as they are global objects.
This could result in security labels for subscriptions not being dumped.

This commit fixes the issue by updating pg_dump to query the pg_seclabels view,
which aggregates entries from both pg_seclabel and pg_shseclabel.
While querying pg_shseclabel directly for subscriptions was an alternative,
using pg_seclabels is simpler and sufficient.

In addition, pg_dump is updated to dump security labels on event triggers,
which were previously omitted.

Backpatch to all supported versions.

Author: Jian He <jian(dot)universality(at)gmail(dot)com>
Co-authored-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/CACJufxHCt00pR9h51AVu6+yPD5J7JQn=7dQXxqacj0XyDhc-fA@mail.gmail.com
Backpatch-through: 13

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/176002c5bfa61421497b382e9a4b4f7315dda42d

Modified Files
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src/bin/pg_dump/pg_backup_archiver.c | 12 +++++++-----
src/bin/pg_dump/pg_dump.c | 7 ++++++-
2 files changed, 13 insertions(+), 6 deletions(-)