pg_catalog: support pg_extension

[jordanlewis]

We should emit an entry for postgis in pg_extension, and likewise for other extensions we might eventually support (e.g. pg_trgrm in #41285).

This prevents the \dx shortcut from psql from working correctly.

Jira issue: CRDB-15687

[otan]

this indeed blocks osm2pgsql: https://github.com/cockroachdb/cockroach/issues/82946#issuecomment-1156811948

wonder if we should backport any additions.

[otan]

Couple of thoughts about this:

• I think the ideal way to implement this is by making CREATE EXTENSION xxx write the extension into the DatabaseDescriptor, and pg_extension should only show entries specifically specified in CREATE EXTENSION xxx. Otherwise we have more rows than necessary in that table. It should also create any functions on the public schema as functions normally do.
• If we want to backport this for the osm case, we could divise a solution where we only show the row if a certain session variable is enabled. This avoids adding a riskier protobuf field.