ACR Tasks Credentials should list keyvault references

**What is the problem you're trying to solve**
When adding credentials to ACR Tasks, we mask the configuration of the username/password in `az acr task show`, which is appropriate for static values.
```
az acr task credential add \
-n node-public \
-r $REGISTRY_PUBLIC \
--login-server $REGISTRY_DOCKERHUB_URL \
-u https://${AKV}.vault.azure.net/secrets/registry-dockerhub-user \
-p https://${AKV}.vault.azure.net/secrets/registry-dockerhub-password \
--use-identity [system]
```
**Describe the solution you'd like**
When configured to keyvault, we should help the customer understand which keyvault secret is configured:
```
az acr task show -r $REGISTRY_PUBLIC -n node-public -o jsonc
{
"agentConfiguration": {
"cpu": 2
},
"agentPoolName": null,
"creationDate": "2020-10-27T17:40:35.307593+00:00",
"credentials": {
"customRegistries": {
"docker.io": {
"username": "https://acr-tasks.vault.azure.net/secrets/registry-dockerhub-user",
"password": "https://acr-tasks.vault.azure.net/secrets/registry-dockerhub-password
}
},
"sourceRegistry": null
```
**Additional context**
Add any other context or screenshots about the feature request here.