azure subnet update remove service-endpoints

> ### `az feedback` auto-generates most of the information requested below, as of CLI version 2.0.62

**Describe the bug**
Existing `service endpoints` were removed if we run the commands without mention every service endpoints.
The blow command will remove all service endpoints in the subnet mention and add only service-endpoint mentions in the command.
```bash
az network vnet subnet update -n $subnet_name -g $network_rg_name --vnet-name $vnet --subscription $subscription_id --service-endpoints Microsoft.ContainerRegistry
```

**To Reproduce**
1. Select vnet
2. Create service endpoint e.g. cosmos DB and select the subnet
3. Run the command to enable another services endpoints e.g. ContainerRegistry
```bash
az network vnet subnet update -n $subnet_name -g $network_rg_name --vnet-name $vnet --subscription $subscription_id --service-endpoints Microsoft.ContainerRegistry
```
4. Existing Service endpoint (Cosmos DB) was deleted.

**Expected behavior**
Existing service endpoints should not be deleted.

**Environment summary**
azure cli
virtual network
service endpoint
OSX