[AVM Module Issue]: Cognitive Service Account - custom RAI policy support

### Check for previous/existing GitHub issues

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

### Issue Type?

Bug

### Module Name

avm/res/cognitive-services/account

### (Optional) Module Version

0.14.1

### Description

The Cognitive Service Account module currently supports only predefined RAI policies via `deployments.raiPolicyName`. Using a custom RAI policy is currently not possible because the RAI policy depends on the Cognitive Service Account resource.

**Expected behavior**

The module should support defining or referencing a custom RAI policy so that it can be used for model deployments within the same module.

**Current behavior / workaround**

As a workaround, the account must be deployed without model deployments. The RAI policy and model deployments then need to be created separately using a custom module. This significantly limits the practical usability of the module.

### (Optional) Correlation Id

_No response_