pg_dump needs SELECT privileges on irrelevant extension table

Hi all,

We have a situation where we need to revoke SELECT on a table that
belongs to our extension, and we also need to let less privileged users
dump the extension's external config tables. (The restricted table's
contents are exposed through a security_barrier view, and it's a cloud
environment where "admin" users don't necessarily have true superuser
access.)

Since the restricted table is internal, its contents aren't included in
dumps anyway, so we expected to be able to meet both use cases at once.
Unfortunately:

    $ pg_dump -U unprivileged_user -d postgres
    pg_dump: error: query failed: ERROR:  permission denied for relation
ext_table
    pg_dump: error: query was: LOCK TABLE public.ext_table IN ACCESS
SHARE MODE

...and there appears to be no way to work around this with
--exclude-table, since the table is part of the extension.

It looks like the only reason pg_dump locks this particular table is
because it's been marked with DUMP_COMPONENT_POLICY, which needs a lock
to ensure the consistency of later pg_get_expr() calls. That stings for
two reasons: 1) it doesn't seem like you need SELECT access on a table
to see its policies, and 2) we have no policies on the table anyway;
there are no pg_get_expr() calls to protect.

So I've attached the simplest backportable workaround I could think of:
unmark DUMP_COMPONENT_POLICY for a table that has no policies at the
time of the getTables() query. This is similar to the ACL optimization
that back branches do; it should ensure that there will be no
pg_get_expr() calls on pg_policy for that table later, due to
repeatable-read, and it omits the lock when there's no reason to grab
it. It won't fix the problem for tables that have do policies, but I
don't have any ideas for how to do that safely, unless there's some lock
mode that uses fewer privileges.

I also attached a speculative backport to 11 to illustrate what that
might look like, but first I have to convince you it's a bug. :)

WDYT?

Thanks,
--Jacob