Add support for PostgreSQL ClickPipes source configuration

[orkhanM]

We're managing a multi-environment infrastructure where we need to:

1. Provision ClickHouse services
2. Set up VPC PrivateLink connectivity to RDS PostgreSQL
3. Create PostgreSQL CDC ClickPipes for real-time data replication
4. All managed declaratively via Terraform

The current clickhouse_clickpipe resource supports creating ClickPipes via Terraform, but lacks proper typing and documentation for PostgreSQL CDC source configurations. While the provider includes clickhouse_clickpipes_reverse_private_endpoint for creating VPC PrivateLink connectivity to PostgreSQL, there's no way to configure the ClickPipe itself to actually use this endpoint for PostgreSQL sources. This makes it difficult to provision end-to-end PostgreSQL CDC ClickPipes with private connectivity in an Infrastructure-as-Code workflow.

[orkhanM]

Closing as it's a duplicate of #329