Show the user objects in the catalog tables

[tamnd]

Part of #3.

The catalog tables now show the objects that a user makes. Before this change pg_class and the other catalogs showed only the built-in rows, so a client could not see a new schema, table, index or sequence.

What changes

• A scan of pg_namespace, pg_class, pg_type, pg_attribute, pg_attrdef, pg_constraint, pg_index, pg_sequence and pg_depend adds the rows of the session catalog after the built-in rows. The new module rupg-exec/src/user.rs makes these rows. The Session trait of rupg-func gives the catalog to the executor.
• CREATE TABLE now makes the TOAST table pg_toast.pg_toast_N and its index, with the same OIDs, columns, operator classes and pg_depend rows as PostgreSQL. The TOAST table holds no data.
• pg_class has relhastriggers and reltoastrelid. A foreign key sets relhastriggers on the two tables. It does not make the triggers yet, so pg_trigger has no rows for them.
• The columns of a sequence are not null, and a unique, primary key or foreign key constraint has connoinherit set, as in PostgreSQL. An index column copies attndims from its table column.
• CREATE INDEX on a TOAST table gives 42501, and a foreign key to a TOAST table gives 42809, with the messages of PostgreSQL.
• xid and cid have text input. xideqint4 and xidneqint4 work, so relfrozenxid = 0 works.

The columns of type pg_node_tree (adbin, conbin, indexprs and indpred) hold the analyzer form of the expression, not the PostgreSQL node text. pg_get_expr and the other deparse functions come in the next PR.

Test

• The new session test makes a schema, two tables with a serial column, a check, a default, a foreign key, an array column and an index on an expression with a predicate. Then it reads the nine catalogs. The expected rows come from the PostgreSQL 19 oracle for the same statements, with the OIDs moved to start at 16384. The pg_node_tree columns are compared as IS NOT NULL, and pg_depend leaves out the rows of the foreign key triggers.
• A new executor test checks that each value of a user row has the kind of its column type, and that a not null column has no NULL.
• The catalog test checks the TOAST table, its index and its pg_depend rows.
• The xid comparisons in the executor test use the results of the oracle.
• srvtest passed on server1.