[Transform] create explicit object mappings for nested field

[hendrikmuhs]

When auto-creating the destination index, transform deducts appropriate mappings, however it creates the mappings only for the leafs, meaning if the field is nested like a.b.c it creates a mapping for a.b.c only. This opens up a corner case:

• a conflicting index template could exists for a or a.b (as reported by a user)

(Note that a conflicting mapping in the config is already handled in validation)

Solution:

• introduce explicit object mappings for nested fields

[elasticmachine]

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