[AVM Module Issue]: Make administrator_login and administrator_login_password optional when azuread_authentication_only_enabled = true

### Check for previous/existing GitHub issues

- [x] I have checked for previous/existing GitHub issues

### Issue Type?

Feature Request

### (Optional) Module Version

0.3.1

### (Optional) Correlation Id

_No response_

### Description

Problem

The AVM module currently requires both:

administrator_login
administrator_login_password

even when:

azuread_authentication_only_enabled = true

In our environment, Azure Policy enforces Microsoft Entra ID-only authentication and disallows SQL authentication.

Although the initial deployment succeeds, subsequent terraform apply operations fail because the module continues to require SQL administrator credentials while the Azure Policy blocks SQL authentication.

Expected behavior

When:

azuread_authentication_only_enabled = true

the module should:

make administrator_login optional
make administrator_login_password optional
avoid configuring SQL administrator credentials entirely

This aligns with Azure SQL behavior, where SQL authentication is disabled when Entra ID-only authentication is enabled.

Use case

Organizations that enforce Microsoft Entra ID-only authentication via Azure Policy cannot reliably use the module because SQL administrator credentials are still required, causing updates to fail even though SQL authentication is intentionally disabled.