az containerapp update - support for init containers

**Related command**
az containerapp update

**Is your feature request related to a problem? Please describe.**
Currently we can update containers in an app by specifying `--container-name`. This does not work for init containers, instead it will create a new app container using the specified name.

**Describe the solution you'd like**
Add the possibility to update an init container alongside with updating an app.

**Additional context**
When creating a new deployment it is a common scenario to have a init container doing tasks like migrations. Ideally this init container have versioned labels to ensure consistency in deployment. This means that it is needed to update the image for both, the app and the init container at the same time.