`az managedapp create` command does not provide an option to control the identity

**Related command**
```
az managedapp create ...
```

**Is your feature request related to a problem? Please describe.**
Similar to issue #21274, there is also no way to set the [identity of the managed application](https://learn.microsoft.com/en-us/azure/templates/microsoft.solutions/applications?pivots=deployment-language-arm-template#identity-1) to be deployed without manually preparing an ARM template.

**Describe the solution you'd like**
Deploying a managed application with only a SystemAssigned identity:
```
az managedapp create ... --identity-system
```

Deploying a managed application with only one or more UserAssigned identities:
```
az managedapp create ... --identity-user /subscription/... /subscription/....
```

Deploying a managed application with both system an user assigned identities:
```
az managedapp create ... --identity-system --identity-user /subscription/... /subscription/....
```

**Describe alternatives you've considered**
Using a regular ARM template with only the resource `Microsoft.Solutions/applications`:
```
az deployment group create ...
```

**Additional context**
Running version azure-cli 2.62.