Added centralized private endpoint for cognitive services account

[bbmilan]

Issue-id

PR Checklist

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• I have added example(s) inside the [./examples/] folder
• I have added the example(s) to the integration test list for normal (~30 minutes) or long runner >30 minutes
• I have checked the coding conventions as per the wiki
• I have checked to ensure there aren't other open Pull Requests for the same update/change?

Description

Does this introduce a breaking change

• YES
• NO

Testing

[arnaudlh]

Thanks for your PR @bbmilan, it looks good to me.
Are you able to take a look at the in-module approach as well as illustrated here: https://github.com/aztfmod/terraform-azurerm-caf/blob/main/modules/redis_cache/private_endpoints.tf
It would be a good addition to have it inside the module also so both centralized (as your PR add) and decentralized (link in example).