feat(db-postgres): support read replicas (#12728)

Adds support for read replicas
https://orm.drizzle.team/docs/read-replicas that can be used to offload
read-heavy traffic.

To use (both `db-postgres` and `db-vercel-postgres` are supported):
```ts
import { postgresAdapter } from '@payloadcms/db-postgres'

database: postgresAdapter({
  pool: {
    connectionString: process.env.POSTGRES_URL,
  },
  readReplicas: [process.env.POSTGRES_REPLICA_URL],
})
```

---------

Co-authored-by: Dan Ribbens <dan.ribbens@gmail.com>

test/database/pg-replica/docker-compose.yaml

@@ -0,0 +1,36 @@
+# Copyright Broadcom, Inc. All Rights Reserved.
+# SPDX-License-Identifier: APACHE-2.0
+
+services:
+  postgresql-master:
+    image: docker.io/bitnami/postgresql:17
+    ports:
+      - '5433:5432'
+    volumes:
+      - 'postgresql_master_data:/bitnami/postgresql'
+    environment:
+      - POSTGRESQL_REPLICATION_MODE=master
+      - POSTGRESQL_REPLICATION_USER=repl_user
+      - POSTGRESQL_REPLICATION_PASSWORD=repl_password
+      - POSTGRESQL_USERNAME=postgres
+      - POSTGRESQL_PASSWORD=my_password
+      - POSTGRESQL_DATABASE=my_database
+      - ALLOW_EMPTY_PASSWORD=yes
+  postgresql-slave:
+    image: docker.io/bitnami/postgresql:17
+    ports:
+      - '5434:5432'
+    depends_on:
+      - postgresql-master
+    environment:
+      - POSTGRESQL_REPLICATION_MODE=slave
+      - POSTGRESQL_REPLICATION_USER=repl_user
+      - POSTGRESQL_REPLICATION_PASSWORD=repl_password
+      - POSTGRESQL_MASTER_HOST=postgresql-master
+      - POSTGRESQL_PASSWORD=my_password
+      - POSTGRESQL_MASTER_PORT_NUMBER=5432
+      - ALLOW_EMPTY_PASSWORD=yes
+
+volumes:
+  postgresql_master_data:
+    driver: local