feat(compute): Introduce privileged_role_name parameter (#12539)

## Problem

Currently `neon_superuser` is hardcoded in many places. It makes it
harder to reuse the same code in different envs.

## Summary of changes

Parametrize `neon_superuser` in `compute_ctl` via
`--privileged-role-name` and in `neon` extensions via
`neon.privileged_role_name`, so it's now possible to use different
'superuser' role names if needed. Everything still defaults to
`neon_superuser`, so no control plane code changes are needed and I
intentionally do not touch regression and migrations tests.

Postgres PRs:
- https://github.com/neondatabase/postgres/pull/674
- https://github.com/neondatabase/postgres/pull/675
- https://github.com/neondatabase/postgres/pull/676
- https://github.com/neondatabase/postgres/pull/677

Cloud PR:
- https://github.com/neondatabase/cloud/pull/31138

compute_tools/src/sql/create_privileged_role.sql

@@ -0,0 +1,8 @@
+DO $$
+    BEGIN
+        IF NOT EXISTS (SELECT FROM pg_catalog.pg_roles WHERE rolname = '{privileged_role_name}')
+        THEN
+            CREATE ROLE {privileged_role_name} CREATEDB CREATEROLE NOLOGIN REPLICATION BYPASSRLS IN ROLE pg_read_all_data, pg_write_all_data;
+        END IF;
+    END
+$$;