feat(vnet)!: per-subnet sourcing and subnet delegation for CloudNGFW

[acelebanski]

Description

This PR introduces 2 new functionalities to the VNET module:

• capability to source existing subnets on a per-subnet basis (not all or nothing like previously)
• subnet delegation to CloudNGFW service (required for VNET-based CloudNGFW deployment)

Motivation and Context

• need for sourcing some of the subnets and creating rest of them within the same VNET
• CloudNGFW support

How Has This Been Tested?

By deploying chosen examples manually.

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes if appropriate.
• All new and existing tests passed.