[Module Proposal]: `avm-res-network-virtualnetworkgateway`

### 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?

Resource Module

### Module Name

avm-res-network-virtualnetworkgateway

### Module Details

Resource module for a virtual network gateway

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

Yes

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

adammontlake

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

_No response_