[Module Proposal]: `avm-ptn-alz-identity`

### 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?

Terraform

### Module Classification?

Pattern Module

### Module Name

avm/ptn/alz/platform-identity

### Module Details

Implementation of the Identity Platform Subscription with Identity VNets and hub peerings aligning with the ALZ architecture.

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

No

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

_No response_

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

_No response_

The proposal names the Terraform module avm/ptn/alz/platform-identity and describes an Identity Platform Subscription with identity VNets and hub peerings aligned to ALZ architecture. No files, tests, or entry points are provided; begin by reviewing the existing module indexes and ALZ architecture guidance. Done means implementing and validating the proposed pattern module against those requirements.

Written by the indexing model from the issue text.