Add ClusterConfigService#getPayload() method
This adds a `getPayload()` method to the `ClusterConfigService` interface. It's needed if you have to convert the config payload on your own and cannot rely on the cluster config to convert it to an object.
## Motivation and Context
Using `@JsonSubTypes` does not work with `ClusterConfigService#get()` because we have to pass the concrete class to the method but the object mapper needs the super type.
An alternative would be to add a `ClusterConfigService#get(type, superType)` method. That would allow me to fix the json subtype usage.
This PR is more generic.
> An alternative would be to add a ClusterConfigService#get(type, superType) method. That would allow me to fix the json subtype usage.
I think I'd prefer that approach and introduce a method like:
T get(String key, Class
Where `key` is the name of the configuration bean (currently the canonical name of the type by default) and `type` is the class it should be cast to.
The old `get(Class
type)` method could then simply delegate to `get(type.getCanonicalName(), type)`.
This would also solve your problem with polymorphic JSON, right?
Updated the PR to use the `get(String key, Class