[Transform] Add a node role to node.transform

[hendrikmuhs]

In #52712 we added node.transform to allow control over placing the transform task to certain nodes up to defining a dedicated transform node.

However we did not add a node role yet. In order to properly document the feature and make it visible in e.g. the cat API's, a node role should be added.

/CC @lcawl

[elasticmachine]

Pinging @elastic/ml-core (:ml/Transform)