Private Link missing policy for some resources.

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### Description

#### Is your feature request related to a problem?

We are trying to exclusively use the policy provided to automatically associate all Private Endpoints to our centralised Private DNS Zones without having to give permissions to our workload Landing Zones. Assumingly there is a few more missing than we have noticed, but we are starting to collate a list of resources that we have noticed as missing:

#### Describe the solution you'd like

- Add support for Postgres - `privatelink.postgres.database.azure.com`

#### Additional context

Inspect the repository for the Private Link policy definitions and existing resource entries, then compare how PostgreSQL private endpoints are represented. Add the missing PostgreSQL resource using the existing policy structure and verify that the policy includes privatelink.postgres.database.azure.com.

Written by the indexing model from the issue text.