[Module Proposal]: `avm-ptn-aiml-ai-gateway`

### 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-aiml-ai-gateway

### Module Details

This module will be the terraform equivalent of the AI Hub Gateway that is currently only available in bicep
https://github.com/Azure-Samples/ai-hub-gateway-solution-accelerator

The module will essentially implement the AI Gateway with foundry modules setup based on the below
https://learn.microsoft.com/en-us/azure/foundry/agents/how-to/ai-gateway?tabs=api-management&pivots=foundry-portal

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

Yes

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

mbilalamjad

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

nadeemis