Default Outbound endpoint and associated subnet for Private DNS Resolver

### Check for previous/existing GitHub issues

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

### Issue Type?

Feature Request

### (Optional) Module Version

v0.15.0

### (Optional) Correlation Id

_No response_

### Description

Hello,

I am just curious if we should enable a way to create a default Outbound endpoint for the Private DNS Resolver and an associated subnet for it, like it's being done for the Inbound endpoint currently.

@jaredfholgate has already given an answer for this here:

https://github.com/Azure/Azure-Landing-Zones/issues/2234#issuecomment-3486250458

But as said there, just creating this one we that we can track it's progress.

Start with the existing implementation for the default Inbound endpoint and review the linked Azure-Landing-Zones discussion for the proposed behavior. Define how a default Outbound endpoint and its associated subnet should be created; done means the feature's scope and expected behavior are agreed and implemented.

Written by the indexing model from the issue text.