Load Balancer Backend Address Pool (2020-05-01): only the designated endpoint is able to manipulate `loadBalancerBackendAddresses`

The [`loadBalancerBackendAddresses`](https://github.com/Azure/azure-rest-api-specs/blob/master/specification/network/resource-manager/Microsoft.Network/stable/2020-05-01/examples/LBBackendAddressPoolWithBackendAddressesPut.json#L42) applies to Standard sku of LB only.

However, if I created a Standard sku LB and try to manipulate it using the [load balancer endpoint](https://github.com/Azure/azure-rest-api-specs/blob/master/specification/network/resource-manager/Microsoft.Network/stable/2020-05-01/loadBalancer.json#L37), it will take no effect.

On the other hand, it works if I send API against the [load balancer backend address pool endpoint](https://github.com/Azure/azure-rest-api-specs/blob/master/specification/network/resource-manager/Microsoft.Network/stable/2020-05-01/loadBalancer.json#L426) instead.

This is unnecessarily complicated from the users' perspective. Because now I have to tell whether the LB of the BAP is standard sku, if so then I can use the *load balancer backend address pool endpoint*, otherwise I will have to use the *load balancer endpoint*.

The expected behavior is:

- Either, allow the *load balancer endpoint* to be able to support the `loadBalancerBackendAddresses`
- Or, allow the *load balancer backend address pool endpoint* to be able to be used for Basic sku

Start by comparing the 2020-05-01 loadBalancer.json definitions at the load balancer endpoint around line 37 and the backend address pool endpoint around line 426. Review the LBBackendAddressPoolWithBackendAddressesPut.json example and determine which endpoint behavior should be made consistent for Standard and Basic SKUs; done means the selected endpoint supports the documented backend-address operation without SKU-dependent endpoint selection.

Written by the indexing model from the issue text.