[Module Proposal]: Azure BuildIn policy assigment

### Check for previous/existing GitHub issues/module proposals

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

### Check this module doesn't already exist in the module indexes

- [x] I have checked for that this module doesn't already exist in the module indexes.

### Bicep or Terraform?

Bicep

### Module Classification?

Resource Module

### Module Name

avm/res/builtin-policy

### Module Details

Module will allow assigne builtin polic and initiative to Managment Group / Subscriptions.

### Do you want to be the owner of this module?

Yes

### Module Owner's GitHub Username (handle)

_No response_

### (Optional) Secondary Module Owner's GitHub Username (handle)

_No response_

The proposal names the AVM module indexes, Bicep, and the target path avm/res/builtin-policy; start by reviewing the indexes and the repository’s Bicep resource-module guidance. No files or tests are specified. Done means defining a module that assigns built-in policies and initiatives to management groups or subscriptions, with implementation details resolved.

Written by the indexing model from the issue text.