fix(aiven_service_integration_endpoint): can't create external_postgresql

[byashimov]

About this change—what it does

This PR covers two related to the issue bugs:

• user config field required for aiven_service_integration_endpoint
• the external_postgresql doesn't have _user_config suffix, which was expected in a few cases. The field name generation is now managed by a single source of truth

Resolves #1589

[byashimov]

tests