[Feature Request]: Add the capability to override the default route to Internet on AzureFirewallSubnet

### Which product(s) are you raising for?

ALZ Terraform (AVM)

### What is the feature request?

I am using the module `avm-ptn-alz-connectivity-hub-and-spoke-vnet` and I would like to be able to have a variable that would allow me to override the default route to Internet to leverage forced tunneling.
I am currently unable to do so because the "internet" route is hardcoded to send traffic to the Internet.

As a workaround, I had to create 2 additionnal routes for 0.0.0.0/1 and 128.0.0.0/1 to send traffic where I need it to go..

### Potential implementation/configuration

```Text

```