ACR agent pool doesn't honor `tags` property during creation

I have following request to create the ACR agent pool:

PUT https://management.azure.com/subscriptions/xxxx/resourceGroups/acctestRG-acragent_pool-230707165015300990/providers/Microsoft.ContainerRegistry/registries/testacccr230707165015300990/agentPools/ap230707165015390?api-version=2019-06-01-preview

```json
{
"location": "westeurope",
"properties": {
"count": 1,
"os": "Linux",
"tier": "S1"
},
"tags": {
"foo": "bar"
}
}
```

Once it's been created successfully (after LRO), I do a GET and have following response body:

```json
{
"type": "Microsoft.ContainerRegistry/registries/agentPools",
"properties": {
"count": 1,
"tier": "S1",
"os": "Linux",
"provisioningState": "Succeeded"
},
"id": "/subscriptions/xxxx/resourceGroups/acctestRG-acragent_pool-230707165015300990/providers/Microsoft.ContainerRegistry/registries/testacccr230707165015300990/agentPools/ap230707165015390",
"name": "ap230707165015390",
"location": "westeurope",
"systemData": {
...
}
}
```

The `tags` specified in the request isn't reflected in the resposne.

## Reference

https://github.com/hashicorp/terraform-provider-azurerm/issues/22397

Start with the ACR agent pool definition for the 2019-06-01-preview API, using the PUT request and subsequent GET response in the report as the reproduction. Check how the top-level tags property is represented and validated, then verify that the documented contract matches the reported create/read behavior.

Written by the indexing model from the issue text.