release-23.1: sql: avoid stampede on synthetic privilege cache in vtable population

[rafiss]

Backport 2/2 commits from #109517.

/cc @cockroachdb/release

Release justification: fix to a performance regression

---

Accessing the synthetic privilege cache requires fetching the descriptor
for system.system_privileges, even in the case of a cache hit. This is
ok for the basic case of fetching a single privilege descriptor for one
virtual table, which is done when executing something like SELECT * FROM a_virtual_table.

However, for some virtual tables, like pg_class, populating the table
itself requires fetching a privilege descriptor for each row in the result.
If a query does a lot of joins, that can mean hundreds of thousands of
calls to the synthetic privilege cache and the descriptor catalog. This
leads to a lot of thrashing in the descriptor catalog, since the catalog
is also being upserted into at the same time, since populating virtual
tables also requires fetching many descriptors.

This can be avoided by some small code changes that short-circuit and
avoid hitting the synthetic privilege cache entirely.

Before:

BenchmarkORMQueries/prisma_column_descriptions-10         	       1	6824922583 ns/op

After:

BenchmarkORMQueries/prisma_column_descriptions-10         	       1	2601088125 ns/op

fixes #108781

Release note (performance improvement): Queries that access the
pg_catalog and information_schema that perform introspection on other
tables in those schemas are now signicantly faster.

Release note (sql change): Introspection queries will now show the
internal node user as the owner of tables in pg_catalog and
information_schema. Previously the owner was shown as admin, but that
was inaccurate since users with the admin role could not modify these
tables in any way.

[cockroach-teamcity]

This change is