Allow serializing aggregation results without typed-keys prefix

[swallez]

Description

The Java API client sends sends the typed_keys=true parameter in _search and similar endpoints to have type information in aggregation and suggestion result names (type#name and not just name).

Similarly, serializing back these objects uses this typed_keys notation. This is an issue when the Java API client is used as a search front-end to ES and results are serialized back go application clients, who may not expect nor understand the type#name format.

The Java API client JsonpMapper should have an option allowing to disable typed_keys notation when serializing objects.