Materialize: Add support for pg_catalog.pg_type

[awang]

npgsql (.net client library) complains about missing the pg_catalog.pg_type table.

I'm putting this in the 0.6 milestone so we can track it, but after thinking about this more, ~I think this is low priority, since it's non-fatal and can be safely ignored~. So I think we should do this after #3727 is complete, and only if it's reasonably straightforward. (Assuming my assumptions remain true).

cc @JLDLaughlin

[JLDLaughlin]

The good news is that this is also required for PgJDBC support, so I'll be working on this soon! (https://github.com/MaterializeInc/materialize/issues/4474, https://github.com/MaterializeInc/materialize/issues/4473, https://github.com/MaterializeInc/materialize/issues/4471, https://github.com/MaterializeInc/materialize/issues/3590)

[awang]

Update -- we think this is a blocker, so we're upgrading the priority of this

[JLDLaughlin]

Talking with Nikhil, this issue is actually blocked by https://github.com/MaterializeInc/materialize/issues/4543

[benesch]

Npgsql and PgJDBC both are very stringent in what they require from pg_type. We're going to temporarily hit pause on this issue to evaluate our options here—mostly by looking at what CockroachDB does. @JLDLaughlin is going to start drafting a design for #4460 in the meantime.