[Load Balancer] Using Subnets instead of VirtualNetworks to link per IP configured in the BackendAddressPool

[memeghaj]

This PR closes #1117

The changes in this PR are as follows:

• Using subnets instead of vnets in the backendAddressPool config of the LoadBalancer.
• Changes to existing builder structs and constructors for backendAddressPool.
• Changes to functionalities to add subnets and link to every IP configured in the backendAddressPool.

I have read the contributing guidelines and have completed the following:

• [x] Tested my code end-to-end against a live Azure subscription.
• [x] Updated the documentation in the docs folder for the affected changes.
• [x] Written unit tests against the modified code that I have made.
• [x] Updated the release notes with a new entry for this PR.
• [x] Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here:

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

backendAddressPool {
      name "lb-backend"
      vnet "my-vnet"
      subnet "my-subnet"
      add_ip_addresses [
          "10.0.1.4"
          "10.0.1.5"
      ]
 }

[ninjarobot]

The option of using a subnet doesn't have to replace the vnet - doing so is a breaking change. Can the builder just add the subnet name so the existing support for using only the vnet remains?

[memeghaj]

The resulting template JSON was tested on a Azure Sub and checked the deployment which was successful.