Manage Fallback to internet feature on linked VNET at Private DNS Zone level

**Related command**
Hi there,

We are using several commands to manage VNET link at Azure Private DNS Zone:

- az network private-dns link vnet create: To create a virtual network link to the specified Private DNS zone,
- az network private-dns link vnet list: To list the virtual network links to the specified private DNS zone,
- az network private-dns link vnet show: To get a virtual network link to the specified private DNS zone,
- az network private-dns link vnet update: To update a virtual network link's properties. Does not modify virtual network within the link.

**Is your feature request related to a problem? Please describe.**
Currently we cannot manage the fallback to internet feature on linked VNET at Private DNS Zone level.

This feature has been announced in February: https://learn.microsoft.com/en-us/azure/dns/private-dns-fallback

**Describe the solution you'd like**
It could be really interesting to be able to enable/update/delete the fallback to internet feature on linked VNET at Private DNS Zone level by adding a new configuration:

**New parameter:** --internet-fallback-enabled
**Accepted values:** 0, 1, f, false, n, no, t, true, y, yes